When a Codex-backed agent is archived from Paseo, call Codex's
thread/archive API so the session is also marked archived in the
Codex app. This keeps both UIs in sync.
- Add optional archiveNativeSession() to AgentClient interface
- Implement for Codex provider via thread/archive JSON-RPC call
- Call best-effort in both archiveAgent() and archiveSnapshot() paths
- Failures are logged as warnings and do not block the archive flow
Closes#822
