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/app-server/src/request_processors/thread_processor.rs

@@ -2671,10 +2671,10 @@ impl ThreadRequestProcessor {
                 )));
             };
 
-            let emit_thread_goal_update = self.config.features.enabled(Feature::Goals);
-            let thread_goal_state_db = emit_thread_goal_update
-                .then(|| self.state_db.clone())
-                .flatten();
+            let (emit_thread_goal_update, thread_goal_state_db) = self
+                .thread_goal_processor
+                .pending_resume_goal_state(existing_thread.as_ref())
+                .await;
 
             let command = crate::thread_state::ThreadListenerCommand::SendThreadResumeResponse(
                 Box::new(crate::thread_state::PendingThreadResumeRequest {