[codex] Inject agent graph store into ThreadManager (#29736)

Pick up the AgentGraphStore migration.

- Inject an explicit optional agent graph store into `ThreadManager` 
- Move all calls to spawn, close, recursive resume, and
subtree/archive/delete/feedback traversal through it
- Keep using  `LocalAgentGraphStore` when SQLite is available

This required some changes to the interface to deal with futures:

- The interface now matches `ThreadStore`'s object-safe pattern by
returning a boxed `AgentGraphStoreFuture` directly, allowing
`ThreadManager` to hold `Arc<dyn AgentGraphStore>`

*Slight behavior change!* Unfiltered subtree enumeration now performs a
single all-status breadth-first traversal, so a closed grandchild
beneath an open edge is included; the previous Open-then-Closed
traversals could not cross mixed-status paths and silently omitted it.

codex-rs/core/src/lib.rs

@@ -118,6 +118,7 @@ pub use thread_manager::StartThreadOptions;
 pub use thread_manager::ThreadManager;
 pub use thread_manager::ThreadShutdownReport;
 pub use thread_manager::build_models_manager;
+pub use thread_manager::local_agent_graph_store_from_state_db;
 pub use thread_manager::thread_store_from_config;
 pub use web_search::web_search_action_detail;
 pub use web_search::web_search_detail;