Add bugfix-pipeline skill - Complete bug fix workflow orchestration

- Level 4 orchestration pipeline
- 5-stage workflow: Investigation → Regression Test → Fix → Verification → Final Check
- Agent coordination (bug-investigator → test-writer → code-reviewer)
- Evidence-based investigation protocol
- Regression test requirements

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

Author: Canato