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/Cargo.lock

@@ -2726,6 +2726,7 @@ dependencies = [
  "codex-context-fragments",
  "codex-core-plugins",
  "codex-core-skills",
+ "codex-diagnostics",
  "codex-exec-server",
  "codex-exec-server-test-support",
  "codex-execpolicy",
@@ -2937,6 +2938,14 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "codex-diagnostics"
+version = "0.0.0"
+dependencies = [
+ "libc",
+ "pretty_assertions",
+]
+
 [[package]]
 name = "codex-exec"
 version = "0.0.0"