core: bundle settings diff updates into one dev/user envelope (#12417)

## Summary
- bundle contextual prompt injection into at most one developer message
plus one contextual user message in both:
  - per-turn settings updates
  - initial context insertion
- preserve `<model_switch>` across compaction by rebuilding it through
canonical initial-context injection, instead of relying on
strip/reattach hacks
- centralize contextual user fragment detection in one shared definition
table and reuse it for parsing/compaction logic
- keep `AGENTS.md` in its natural serialized format:
  - `# AGENTS.md instructions for {dirname}`
  - `<INSTRUCTIONS>...</INSTRUCTIONS>`
- simplify related tests/helpers and accept the expected snapshot/layout
updates from bundled multi-part messages

## Why
The goal is to converge toward a simpler, more intentional prompt shape
where contextual updates are consistently represented as one developer
envelope plus one contextual user envelope, while keeping parsing and
compaction behavior aligned with that representation.

## Notable details
- the temporary `SettingsUpdateEnvelope` wrapper was removed; these
paths now return `Vec<ResponseItem>` directly
- local/remote compaction no longer rely on model-switch strip/restore
helpers
- contextual user detection is now driven by shared fragment definitions
instead of ad hoc matcher assembly
- AGENTS/user instructions are still the same logical context; only the
synthetic `<user_instructions>` wrapper was replaced by the natural
AGENTS text format

## Testing
- `just fmt`
- `cargo test -p codex-app-server
codex_message_processor::tests::extract_conversation_summary_prefers_plain_user_messages
-- --exact`
- `cargo test -p codex-core
compact::tests::collect_user_messages_filters_session_prefix_entries
--lib -- --exact`
- `cargo test -p codex-core --test all
'suite::compact::snapshot_request_shape_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::compact_remote::snapshot_request_shape_remote_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_apps_guidance_as_developer_message_when_enabled'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_developer_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::includes_user_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::resume_includes_initial_messages_and_sends_prior_items'
-- --exact`
- `cargo test -p codex-core --test all
'suite::review::review_input_isolated_from_parent_history' -- --exact`
- `cargo test -p codex-exec --test all
'suite::resume::exec_resume_last_respects_cwd_filter_and_all_flag' --
--exact`
- `cargo test -p core_test_support
context_snapshot::tests::full_text_mode_preserves_unredacted_text --
--exact`

## Notes
- I also ran several targeted `compact`, `compact_remote`,
`prompt_caching`, `model_visible_layout`, and `event_mapping` tests
while iterating on prompt-shape changes.
- I have not claimed a clean full-workspace `cargo test` from this
environment because local sandbox/resource conditions have previously
produced unrelated failures in large workspace runs.

codex-rs/core/src/environment_context.rs

@@ -1,9 +1,7 @@
 use crate::codex::TurnContext;
+use crate::contextual_user_message::ENVIRONMENT_CONTEXT_FRAGMENT;
 use crate::shell::Shell;
-use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
-use codex_protocol::protocol::ENVIRONMENT_CONTEXT_CLOSE_TAG;
-use codex_protocol::protocol::ENVIRONMENT_CONTEXT_OPEN_TAG;
 use codex_protocol::protocol::TurnContextItem;
 use codex_protocol::protocol::TurnContextNetworkItem;
 use serde::Deserialize;
@@ -40,7 +38,6 @@ impl EnvironmentContext {
         let EnvironmentContext {
             cwd,
             network,
-            // should compare all fields except shell
             shell: _,
         } = other;
         self.cwd == *cwd && self.network == *network
@@ -122,7 +119,7 @@ impl EnvironmentContext {
     /// </environment_context>
     /// ```
     pub fn serialize_to_xml(self) -> String {
-        let mut lines = vec![ENVIRONMENT_CONTEXT_OPEN_TAG.to_string()];
+        let mut lines = Vec::new();
         if let Some(cwd) = self.cwd {
             lines.push(format!("  <cwd>{}</cwd>", cwd.to_string_lossy()));
         }
@@ -145,22 +142,13 @@ impl EnvironmentContext {
                 // lines.push("  <network enabled=\"false\" />".to_string());
             }
         }
-        lines.push(ENVIRONMENT_CONTEXT_CLOSE_TAG.to_string());
-        lines.join("\n")
+        ENVIRONMENT_CONTEXT_FRAGMENT.wrap(lines.join("\n"))
     }
 }
 
 impl From<EnvironmentContext> for ResponseItem {
     fn from(ec: EnvironmentContext) -> Self {
-        ResponseItem::Message {
-            id: None,
-            role: "user".to_string(),
-            content: vec![ContentItem::InputText {
-                text: ec.serialize_to_xml(),
-            }],
-            end_turn: None,
-            phase: None,
-        }
+        ENVIRONMENT_CONTEXT_FRAGMENT.into_message(ec.serialize_to_xml())
     }
 }