CLI: add trace diff command for comparing two sessions

When debugging regressions, you want to compare a good run vs a bad run side by side:

```bash
ai-trace diff session_good.jsonl session_bad.jsonl
```

Output should highlight:
- Steps that diverged (different decisions on same input)
- Timing differences (which step got slower)
- Missing/extra steps

Useful for postmortems and A/B testing agent behavior.