## 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.
