Error Codes + Envelope

Error envelope

{
  "ok": false,
  "data": null,
  "meta": {
    "requestId": "req_123",
    "sessionId": "sess_123",
    "executionId": "exec_123",
    "timingMs": {
      "total": 42,
      "policy": 12,
      "route": 8,
      "tool": 10
    }
  },
  "error": {
    "code": "POLICY_DENIED",
    "message": "Policy denied execution.",
    "status": 403,
    "retryable": false,
    "details": {
      "rule": "pii_block"
    }
  }
}