[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/agent/control.rs

@@ -36,7 +36,6 @@ use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::ThreadSource;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_protocol::user_input::UserInput;
-use codex_state::DirectionalThreadSpawnEdgeStatus;
 use codex_thread_store::ReadThreadParams;
 use serde::Serialize;
 use std::collections::HashMap;
@@ -639,7 +638,7 @@ impl AgentControl {
 
     async fn persist_thread_spawn_edge_for_source(
         &self,
-        thread: &crate::CodexThread,
+        child_thread: &crate::CodexThread,
         child_thread_id: ThreadId,
         session_source: Option<&SessionSource>,
     ) {
@@ -647,14 +646,20 @@ impl AgentControl {
         else {
             return;
         };
-        let Some(state_db_ctx) = thread.state_db() else {
+        if child_thread.config_snapshot().await.ephemeral {
+            return;
+        }
+        let Ok(state) = self.upgrade() else {
             return;
         };
-        if let Err(err) = state_db_ctx
+        let Some(agent_graph_store) = state.agent_graph_store() else {
+            return;
+        };
+        if let Err(err) = agent_graph_store
             .upsert_thread_spawn_edge(
                 parent_thread_id,
                 child_thread_id,
-                DirectionalThreadSpawnEdgeStatus::Open,
+                codex_agent_graph_store::ThreadSpawnEdgeStatus::Open,
             )
             .await
         {