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/diagnostics/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "codex-diagnostics"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+name = "codex_diagnostics"
+path = "src/lib.rs"
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+libc = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }