* fix(gateway): validate PID ownership and clean stale pid files
- include `pid` in health responses for runtime PID verification
- add `RemovePidFileIfPID` to safely delete PID files only on PID match
- sanitize gateway PID data via process-command checks with health fallback
- ignore and remove stale/non-gateway PID files before gateway operations
- refuse stop/restart actions when the attached process is not a gateway
- update gateway and websocket tests to cover PID validation and safety paths
* test(seahorse): use shared in-memory SQLite DB in tests to fix async compaction failures
* test: remove unused sendMediaErr field from hook test mock
- treat `EPERM` from `signal(0)` as “process exists” on Unix
- classify malformed PID files as invalid and auto-remove them during read
- keep cached `pidData` only for transient races and downgrade `running` to `stopped` when the tracked process is gone
- refresh PID data on WebSocket proxy requests and reject stale cached gateway state
- add regression tests for invalid PID files, status downgrade, on-demand PID loading, and stale proxy rejection
