Add bug-investigator agent - Systematic bug diagnosis

- READ-ONLY agent with investigation protocol
- Reproduce FIRST (never investigate without reproduction)
- Evidence quality ranking (controlled > observed > speculation)
- 6-step investigation protocol with circuit breaker
- Structured report template with root cause analysis

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Canato