Revert state DB injection and agent graph store (#21481)

## Why

Reverts #20689 to restore the previous optional state DB plumbing. The
conflict resolution keeps the newer installation ID and session/thread
identity changes that landed after #20689, while removing the mandatory
state DB and agent graph store dependency from ThreadManager
construction.

## What changed

- Restored `Option<StateDbHandle>` through app-server, MCP server,
prompt debug, and test entry points.
- Removed the `codex-core` dependency on `codex-agent-graph-store` and
reverted descendant lookup back to the existing state DB path when
available.
- Kept newer `installation_id` forwarding by passing it beside the
optional DB handle.
- Kept local thread-name updates working when the optional state DB
handle is absent.

## Validation

- `git diff --check`
- `cargo test -p codex-thread-store`
- `cargo test -p codex-state -p codex-rollout -p
codex-app-server-protocol`
- Attempted `env CARGO_INCREMENTAL=0 cargo test -p codex-core -p
codex-app-server -p codex-app-server-client -p codex-mcp-server -p
codex-thread-manager-sample -p codex-tui`; blocked locally by a rustc
ICE while compiling `v8 v146.4.0` with `rustc 1.93.0 (254b59607
2026-01-19)` on `aarch64-apple-darwin`.

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

@@ -132,6 +132,7 @@ use codex_terminal_detection::user_agent;
 use codex_thread_store::CreateThreadParams;
 use codex_thread_store::LiveThread;
 use codex_thread_store::LiveThreadInitGuard;
+use codex_thread_store::LocalThreadStore;
 use codex_thread_store::ResumeThreadParams;
 use codex_thread_store::ThreadEventPersistenceMode;
 use codex_thread_store::ThreadPersistenceMetadata;
@@ -352,6 +353,7 @@ use codex_protocol::protocol::TokenUsage;
 use codex_protocol::protocol::TokenUsageInfo;
 use codex_protocol::protocol::WarningEvent;
 use codex_protocol::user_input::UserInput;
+use codex_tools::ToolEnvironmentMode;
 use codex_tools::ToolsConfig;
 use codex_tools::ToolsConfigParams;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -409,7 +411,6 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) parent_trace: Option<W3cTraceContext>,
     pub(crate) environment_selections: ResolvedTurnEnvironments,
     pub(crate) analytics_events_client: Option<AnalyticsEventsClient>,
-    pub(crate) state_db: Option<state_db::StateDbHandle>,
     pub(crate) thread_store: Arc<dyn ThreadStore>,
 }
 
@@ -469,7 +470,6 @@ impl Codex {
             parent_trace: _,
             environment_selections,
             analytics_events_client,
-            state_db,
             thread_store,
         } = args;
         let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
@@ -558,7 +558,15 @@ impl Codex {
             };
             match thread_id {
                 Some(thread_id) => {
-                    let state_db_ctx = state_db.clone();
+                    let state_db_ctx = if config.ephemeral {
+                        None
+                    } else if let Some(local_store) =
+                        thread_store.as_any().downcast_ref::<LocalThreadStore>()
+                    {
+                        local_store.state_db().await
+                    } else {
+                        None
+                    };
                     state_db::get_dynamic_tools(state_db_ctx.as_deref(), thread_id, "codex_spawn")
                         .await
                 }
@@ -646,7 +654,6 @@ impl Codex {
             agent_control,
             environment_manager,
             analytics_events_client,
-            state_db,
             thread_store,
             parent_rollout_thread_trace,
         )
@@ -1308,7 +1315,7 @@ impl Session {
             self.services.user_shell.as_ref().clone(),
             self.services.shell_snapshot_tx.clone(),
             self.services.session_telemetry.clone(),
-            self.state_db(),
+            self.services.state_db.clone(),
         );
     }