Add process diagnostics snapshots (#37434)

## 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

codex-rs/core/src/thread_manager_tests.rs

@@ -800,6 +800,12 @@ async fn start_thread_keeps_internal_threads_hidden_from_normal_lookups() {
 
     assert_eq!(manager.list_thread_ids().await, Vec::new());
     assert!(manager.get_thread(thread.thread_id).await.is_err());
+    assert!(
+        codex_diagnostics::snapshot()
+            .gauges
+            .iter()
+            .any(|gauge| gauge.name == "core.threads.live" && gauge.value > 0)
+    );
 
     let report = manager
         .shutdown_all_threads_bounded(Duration::from_secs(10))