## What changed
- Add a `codex-diagnostics` crate that snapshots the process ID, available
resident-memory measurements, and registered process-wide gauges.
- Provide guards that update gauges for the lifetime of measured objects.
- Track live `CodexThread` instances with the `core.threads.live` gauge.
## Testing
- Add unit coverage for gauge registration, guard lifetimes, process memory
snapshots, and live-thread reporting.
GitOrigin-RevId: 3236b086bd4ebe31ed4768ab87a5fa288b0891b0
