feat: add Axiom telemetry for PR review pipeline

- New telemetry module (github_app/telemetry.py) sends structured events to Axiom
- Instrument worker.py (webhook path) and app.py (API path) with timing + metrics
- ReviewMetrics dataclass tracks tool usage, API calls, and loop rounds
- metrics_out parameter flows metrics from reviewer to callers
- Add axiom-py dependency to both services
- Graceful no-op when AXIOM_TOKEN is not set

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>