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/tests/guardian_tests.rs

@@ -731,12 +731,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
     let skills_watcher = Arc::new(SkillsWatcher::noop());
     let thread_store = Arc::new(codex_thread_store::LocalThreadStore::new(
         codex_thread_store::LocalThreadStoreConfig::from_config(&config),
-        codex_state::StateRuntime::init(
-            config.sqlite_home.clone(),
-            config.model_provider_id.clone(),
-        )
-        .await
-        .expect("state db should initialize"),
+        /*state_db*/ None,
     ));
 
     let CodexSpawnOk { codex, .. } = Codex::spawn(CodexSpawnArgs {
@@ -767,7 +762,6 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
             turn_environments: Vec::new(),
         },
         analytics_events_client: None,
-        state_db: None,
         thread_store,
     })
     .await