Add plugin-attributed skill telemetry (#40724)

## What changed

- Add `plugin_id`, `model_slug`, and `reasoning_effort` dimensions to
  `codex.skill.injected` metrics for explicit and implicit skill invocations.
- Propagate plugin IDs from orchestrator skill metadata so resource-backed skill
  invocations can emit the same attribution.
- Record `codex.skill.turn.duration_seconds` once per plugin used in a turn,
  tagged with the model, reasoning effort, and completed, aborted, or error
  status.

GitOrigin-RevId: d249a9c4e85682b1106ac49d9e598535c6cbb06d