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/goals.rs

@@ -30,6 +30,7 @@ use codex_protocol::protocol::TokenUsage;
 use codex_protocol::protocol::TurnAbortReason;
 use codex_protocol::protocol::validate_thread_goal_objective;
 use codex_rollout::state_db::reconcile_rollout;
+use codex_thread_store::LocalThreadStore;
 use codex_utils_template::Template;
 use futures::future::BoxFuture;
 use std::sync::Arc;
@@ -1338,6 +1339,17 @@ impl Session {
             state_db
         } else if let Some(state_db) = self.goal_runtime.state_db.lock().await.clone() {
             state_db
+        } else if let Some(local_store) = self
+            .services
+            .thread_store
+            .as_any()
+            .downcast_ref::<LocalThreadStore>()
+        {
+            local_store.state_db().await.ok_or_else(|| {
+                anyhow::anyhow!(
+                    "thread goals require a local persisted thread with a state database"
+                )
+            })?
         } else {
             anyhow::bail!("thread goals require a local persisted thread with a state database");
         };