[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/Cargo.toml

@@ -504,7 +504,6 @@ unwrap_used = "deny"
 # silence the false positive here instead of deleting a real dependency.
 [workspace.metadata.cargo-shear]
 ignored = [
-    "codex-agent-graph-store",
     "icu_provider",
     "openssl-sys",
     "codex-v8-poc",