feat: detailed Axiom telemetry events throughout review pipeline

Event types emitted:
- review.started / review.completed / review.failed — lifecycle
- review.clone / review.post — outer operations
- review.agentic_round — each loop round with tools used
- review.tool_call — each tool execution with timing, input, result
- review.api_call — each Claude API call with token counts
- review.extraction — structured extraction step
- review.warpgrep_failed — WarpGrep-specific warnings

Uses callback pattern (TelemetryCallback) to decouple reviewer.py
from Axiom. make_event_emitter() factory auto-attaches base context.
Token totals tracked in ReviewMetrics and propagated to callers.

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