core: remove redundant TurnContext and Prompt fields (#28638)

## Why

`TurnContext` had accumulated dead fields and cached projections of
values already owned by its per-turn `Config` or `ModelInfo`. Keeping
both copies made ownership unclear and allowed artificial split-brain
states, such as a compatibility hash differing from the model metadata
it came from.

`Prompt` similarly carried a write-only personality after personality
selection had already been materialized into its base instructions.

This makes the canonical owner explicit: configuration-backed values
come from `config`, model-derived values come from `model_info`, and
prompts contain only data consumed by request construction.

## What changed

- Remove the unused `ghost_snapshot`, `codex_self_exe`, and
`thread_source` fields.
- Remove duplicate `comp_hash`, `truncation_policy`, `features`,
`shell_environment_policy`, `codex_linux_sandbox_exe`, `compact_prompt`,
and `tool_mode` fields.
- Read those values directly from `TurnContext::config` or
`TurnContext::model_info` at their consumers.
- Remove the write-only `Prompt::personality` field and its constructor
assignments.
- Preserve review-turn inheritance of the parent turn's shell policy,
Linux sandbox executable, and compact prompt through the review config.

## Testing

- `cargo check -p codex-core --tests`

codex-rs/core/src/session/turn_context.rs

@@ -1,7 +1,6 @@
 use super::*;
 use crate::SkillLoadOutcome;
 use crate::agents_md::LoadedAgentsMd;
-use crate::config::GhostSnapshotConfig;
 use crate::environment_selection::TurnEnvironmentSnapshot;
 use crate::shell_snapshot::ShellSnapshotFile;
 use codex_core_skills::HostLoadedSkills;
@@ -12,9 +11,7 @@ use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::openai_models::ModelInfo;
-use codex_protocol::openai_models::ToolMode;
 use codex_protocol::protocol::MultiAgentVersion;
-use codex_protocol::protocol::ThreadSource;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_sandboxing::compatibility_sandbox_policy_for_permission_profile;
 use codex_sandboxing::policy_transforms::effective_file_system_sandbox_policy;
@@ -111,15 +108,12 @@ pub struct TurnContext {
     pub config: Arc<Config>,
     pub(crate) auth_manager: Option<Arc<AuthManager>>,
     pub(crate) model_info: ModelInfo,
-    pub(crate) comp_hash: Option<String>,
-    pub(crate) tool_mode: ToolMode,
     pub(crate) session_telemetry: SessionTelemetry,
     pub(crate) provider: SharedModelProvider,
     pub(crate) reasoning_effort: Option<ReasoningEffortConfig>,
     pub(crate) reasoning_summary: ReasoningSummaryConfig,
     pub(crate) session_source: SessionSource,
     pub(crate) parent_thread_id: Option<ThreadId>,
-    pub(crate) thread_source: Option<ThreadSource>,
     pub(crate) environments: TurnEnvironmentSnapshot,
     /// The session's absolute working directory. All relative paths provided
     /// by the model as well as sandbox policies are resolved against this path
@@ -130,7 +124,6 @@ pub struct TurnContext {
     pub(crate) timezone: Option<String>,
     pub(crate) app_server_client_name: Option<String>,
     pub(crate) developer_instructions: Option<String>,
-    pub(crate) compact_prompt: Option<String>,
     pub(crate) user_instructions: Option<String>,
     pub(crate) collaboration_mode: CollaborationMode,
     pub(crate) multi_agent_version: MultiAgentVersion,
@@ -139,15 +132,9 @@ pub struct TurnContext {
     pub(crate) permission_profile: PermissionProfile,
     pub(crate) network: Option<NetworkProxy>,
     pub(crate) windows_sandbox_level: WindowsSandboxLevel,
-    pub(crate) shell_environment_policy: ShellEnvironmentPolicy,
     pub(crate) available_models: Vec<ModelPreset>,
     pub(crate) unified_exec_shell_mode: UnifiedExecShellMode,
-    pub features: ManagedFeatures,
-    pub(crate) ghost_snapshot: GhostSnapshotConfig,
     pub(crate) final_output_json_schema: Option<Value>,
-    pub(crate) codex_self_exe: Option<PathBuf>,
-    pub(crate) codex_linux_sandbox_exe: Option<PathBuf>,
-    pub(crate) truncation_policy: TruncationPolicy,
     pub(crate) dynamic_tools: Vec<DynamicToolSpec>,
     pub(crate) turn_metadata_state: Arc<TurnMetadataState>,
     pub(crate) extension_data: Arc<codex_extension_api::ExtensionData>,
@@ -214,7 +201,9 @@ impl TurnContext {
             .auth_manager
             .as_deref()
             .is_some_and(AuthManager::current_auth_uses_codex_backend);
-        self.features.apps_enabled_for_auth(uses_codex_backend)
+        self.config
+            .features
+            .apps_enabled_for_auth(uses_codex_backend)
     }
 
     pub(crate) fn tool_environment_mode(&self) -> ToolEnvironmentMode {
@@ -231,16 +220,6 @@ impl TurnContext {
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
-        let tool_mode = model_info.tool_mode.unwrap_or_else(|| {
-            if config.features.enabled(Feature::CodeModeOnly) {
-                ToolMode::CodeModeOnly
-            } else if config.features.enabled(Feature::CodeMode) {
-                ToolMode::CodeMode
-            } else {
-                ToolMode::Direct
-            }
-        });
-        let truncation_policy = model_info.truncation_policy.into();
         let supported_reasoning_levels = model_info
             .supported_reasoning_levels
             .iter()
@@ -269,7 +248,6 @@ impl TurnContext {
             Some(reasoning_effort.clone()),
             /*developer_instructions*/ None,
         );
-        let features = self.features.clone();
         let available_models = models_manager
             .list_models(RefreshStrategy::OnlineIfUncached)
             .await;
@@ -281,8 +259,6 @@ impl TurnContext {
             config: Arc::new(config),
             auth_manager: self.auth_manager.clone(),
             model_info: model_info.clone(),
-            comp_hash: model_info.comp_hash.clone(),
-            tool_mode,
             session_telemetry: self
                 .session_telemetry
                 .clone()
@@ -292,7 +268,6 @@ impl TurnContext {
             reasoning_summary: self.reasoning_summary,
             session_source: self.session_source.clone(),
             parent_thread_id: self.parent_thread_id,
-            thread_source: self.thread_source.clone(),
             environments: self.environments.clone(),
             #[allow(deprecated)]
             cwd: self.cwd.clone(),
@@ -300,7 +275,6 @@ impl TurnContext {
             timezone: self.timezone.clone(),
             app_server_client_name: self.app_server_client_name.clone(),
             developer_instructions: self.developer_instructions.clone(),
-            compact_prompt: self.compact_prompt.clone(),
             user_instructions: self.user_instructions.clone(),
             collaboration_mode,
             multi_agent_version: self.multi_agent_version,
@@ -309,15 +283,9 @@ impl TurnContext {
             permission_profile: self.permission_profile.clone(),
             network: self.network.clone(),
             windows_sandbox_level: self.windows_sandbox_level,
-            shell_environment_policy: self.shell_environment_policy.clone(),
             available_models,
             unified_exec_shell_mode: self.unified_exec_shell_mode.clone(),
-            features,
-            ghost_snapshot: self.ghost_snapshot.clone(),
             final_output_json_schema: self.final_output_json_schema.clone(),
-            codex_self_exe: self.codex_self_exe.clone(),
-            codex_linux_sandbox_exe: self.codex_linux_sandbox_exe.clone(),
-            truncation_policy,
             dynamic_tools: self.dynamic_tools.clone(),
             turn_metadata_state: self.turn_metadata_state.clone(),
             extension_data: Arc::clone(&self.extension_data),
@@ -368,7 +336,7 @@ impl TurnContext {
                 .config
                 .permissions
                 .windows_sandbox_private_desktop,
-            use_legacy_landlock: self.features.use_legacy_landlock(),
+            use_legacy_landlock: self.config.features.use_legacy_landlock(),
         }
     }
 
@@ -388,12 +356,6 @@ impl TurnContext {
             .then_some(file_system_sandbox_policy)
     }
 
-    pub(crate) fn compact_prompt(&self) -> &str {
-        self.compact_prompt
-            .as_deref()
-            .unwrap_or(compact::SUMMARIZATION_PROMPT)
-    }
-
     pub(crate) fn to_turn_context_item(&self) -> TurnContextItem {
         let workspace_roots = self.config.effective_workspace_roots();
         #[allow(deprecated)]
@@ -410,7 +372,7 @@ impl TurnContext {
             network: self.turn_context_network_item(),
             file_system_sandbox_policy: self.non_legacy_file_system_sandbox_policy(),
             model: self.model_info.slug.clone(),
-            comp_hash: self.comp_hash.clone(),
+            comp_hash: self.model_info.comp_hash.clone(),
             personality: self.personality,
             collaboration_mode: Some(self.collaboration_mode.clone()),
             multi_agent_version: Some(self.multi_agent_version),
@@ -549,15 +511,6 @@ impl Session {
         );
 
         let mut per_turn_config = per_turn_config;
-        let tool_mode = model_info.tool_mode.unwrap_or_else(|| {
-            if per_turn_config.features.enabled(Feature::CodeModeOnly) {
-                ToolMode::CodeModeOnly
-            } else if per_turn_config.features.enabled(Feature::CodeMode) {
-                ToolMode::CodeMode
-            } else {
-                ToolMode::Direct
-            }
-        });
         per_turn_config.service_tier = get_service_tier(
             per_turn_config.service_tier,
             per_turn_config.features.enabled(Feature::FastMode),
@@ -583,18 +536,15 @@ impl Session {
             sub_id,
             trace_id: current_span_trace_id(),
             realtime_active: false,
-            config: per_turn_config.clone(),
+            config: per_turn_config,
             auth_manager: auth_manager_for_context,
-            model_info: model_info.clone(),
-            comp_hash: model_info.comp_hash.clone(),
-            tool_mode,
+            model_info,
             session_telemetry: session_telemetry_for_context,
             provider: provider_for_context,
             reasoning_effort,
             reasoning_summary,
             session_source,
             parent_thread_id: session_configuration.parent_thread_id,
-            thread_source: session_configuration.thread_source.clone(),
             environments,
             #[allow(deprecated)]
             cwd,
@@ -602,7 +552,6 @@ impl Session {
             timezone: Some(timezone),
             app_server_client_name: session_configuration.app_server_client_name.clone(),
             developer_instructions: session_configuration.developer_instructions.clone(),
-            compact_prompt: session_configuration.compact_prompt.clone(),
             user_instructions: session_configuration
                 .loaded_agents_md
                 .as_ref()
@@ -614,15 +563,9 @@ impl Session {
             permission_profile: session_configuration.permission_profile(),
             network,
             windows_sandbox_level: session_configuration.windows_sandbox_level,
-            shell_environment_policy: per_turn_config.permissions.shell_environment_policy.clone(),
             available_models,
             unified_exec_shell_mode,
-            features: per_turn_config.features.clone(),
-            ghost_snapshot: per_turn_config.ghost_snapshot.clone(),
             final_output_json_schema: None,
-            codex_self_exe: per_turn_config.codex_self_exe.clone(),
-            codex_linux_sandbox_exe: per_turn_config.codex_linux_sandbox_exe.clone(),
-            truncation_policy: model_info.truncation_policy.into(),
             dynamic_tools: session_configuration.dynamic_tools.clone(),
             turn_metadata_state,
             extension_data,