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/Cargo.lock

@@ -1748,6 +1748,21 @@ dependencies = [
  "unicode-width 0.2.1",
 ]
 
+[[package]]
+name = "codex-agent-graph-store"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "codex-protocol",
+ "codex-state",
+ "pretty_assertions",
+ "serde",
+ "serde_json",
+ "tempfile",
+ "thiserror 2.0.18",
+ "tokio",
+]
+
 [[package]]
 name = "codex-agent-identity"
 version = "0.0.0"