Add agent graph store interface (#19229)

## Summary

Persisted subagent parent/child topology currently leaks through
`StateRuntime`'s SQLite-specific thread-spawn helpers. This PR
introduces a narrow `AgentGraphStore` boundary so follow-up work can
route graph operations through a local or remote store without coupling
orchestration code directly to the state DB graph API.

## Changes

- Adds the new `codex-agent-graph-store` crate.
- Defines a flat `AgentGraphStore` trait for the v1 graph surface:
upsert edge, set edge status, list direct children, and list
descendants.
- Adds public graph types for `ThreadSpawnEdgeStatus`,
`AgentGraphStoreError`, and `AgentGraphStoreResult`.
- Implements `LocalAgentGraphStore` on top of an existing
`codex_state::StateRuntime`, preserving today's SQLite-backed
`thread_spawn_edges` behavior.
- Registers the crate in Cargo/Bazel metadata.

This PR only adds the local contract and implementation; call-site
migration and the remote gRPC store are left to the follow-up PRs in the
stack.

## Testing

- `cargo test -p codex-agent-graph-store`

The new unit tests cover local parity with the existing `StateRuntime`
graph methods, `Open`/`Closed` filtering, status updates, and stable
breadth-first descendant ordering.

codex-rs/state/src/runtime/threads.rs

@@ -134,6 +134,15 @@ ON CONFLICT(child_thread_id) DO UPDATE SET
             .await
     }
 
+    /// List all direct spawned children of `parent_thread_id`.
+    pub async fn list_thread_spawn_children(
+        &self,
+        parent_thread_id: ThreadId,
+    ) -> anyhow::Result<Vec<ThreadId>> {
+        self.list_thread_spawn_children_matching(parent_thread_id, /*status*/ None)
+            .await
+    }
+
     /// List spawned descendants of `root_thread_id` whose edges match `status`.
     ///
     /// Descendants are returned breadth-first by depth, then by thread id for stable ordering.
@@ -1871,4 +1880,65 @@ mod tests {
             .expect("all descendants should load");
         assert_eq!(all_descendants, vec![child_thread_id, grandchild_thread_id]);
     }
+
+    #[tokio::test]
+    async fn thread_spawn_children_without_status_filter_lists_all_statuses() {
+        let codex_home = unique_temp_dir();
+        let runtime = StateRuntime::init(codex_home, "test-provider".to_string())
+            .await
+            .expect("state db should initialize");
+        let parent_thread_id =
+            ThreadId::from_string("00000000-0000-0000-0000-000000000910").expect("valid thread id");
+        let open_child_thread_id =
+            ThreadId::from_string("00000000-0000-0000-0000-000000000911").expect("valid thread id");
+        let closed_child_thread_id =
+            ThreadId::from_string("00000000-0000-0000-0000-000000000912").expect("valid thread id");
+        let future_child_thread_id =
+            ThreadId::from_string("00000000-0000-0000-0000-000000000913").expect("valid thread id");
+
+        runtime
+            .upsert_thread_spawn_edge(
+                parent_thread_id,
+                open_child_thread_id,
+                DirectionalThreadSpawnEdgeStatus::Open,
+            )
+            .await
+            .expect("open child edge insert should succeed");
+        runtime
+            .upsert_thread_spawn_edge(
+                parent_thread_id,
+                closed_child_thread_id,
+                DirectionalThreadSpawnEdgeStatus::Closed,
+            )
+            .await
+            .expect("closed child edge insert should succeed");
+        sqlx::query(
+            r#"
+INSERT INTO thread_spawn_edges (
+    parent_thread_id,
+    child_thread_id,
+    status
+) VALUES (?, ?, ?)
+            "#,
+        )
+        .bind(parent_thread_id.to_string())
+        .bind(future_child_thread_id.to_string())
+        .bind("future")
+        .execute(runtime.pool.as_ref())
+        .await
+        .expect("future-status child edge insert should succeed");
+
+        let children = runtime
+            .list_thread_spawn_children(parent_thread_id)
+            .await
+            .expect("all children should load");
+        assert_eq!(
+            children,
+            vec![
+                open_child_thread_id,
+                closed_child_thread_id,
+                future_child_thread_id,
+            ]
+        );
+    }
 }