feat(ci): add TDD bug-fix agent and triage cron

[peppescg]

Summary

Adds automated bug fixing using a TDD approach with Claude Code agents:

• Bug-fix agent (_bug-fix-agent.yml): Triggered by auto-fix label on Bug issues. Two-phase TDD — Opus writes a failing test (red), Sonnet applies the minimum fix (green). Up to 3 attempts with 5 internal retries per phase.
• Trigger workflow (bug-fix-on-label.yml): Fires when auto-fix label is added to a Bug issue.
• Triage cron (bug-triage-cron.yml): Daily (weekdays 8am UTC) Sonnet evaluates open Bug issues and labels suitable ones with auto-fix (max 3/day).
• TDD skill (.claude/skills/bug-fix-tdd/): Test patterns, placement rules, and workflow instructions for the agent.

Key design decisions

• Only comments on issue after all 3 attempts are exhausted (no noise on intermediate failures)
• Skips issues with human comments, cross-repo fixes (timeline API), and existing PRs
• Hard gates between phases verify test actually fails/passes (not just agent's word)
• continue-on-error on TDD step ensures failure comment posts even on timeout
• Expression injection mitigated by using env vars instead of ${{ }} in run blocks

Configuration

Setting	Value
Bug-fix timeout	45 min
Triage timeout	15 min
Max attempts per issue	3
Max internal retries per phase	5
Phase 1 (Opus) max turns	50
Phase 2 (Sonnet) max turns	80
Triage (Sonnet) max turns	20
Triage max issues/day	3
Triage schedule	Weekdays 08:00 UTC

Test plan

• Create a test Bug issue with clear reproduction steps, add auto-fix label, verify agent creates PR
• Create a vague Bug issue, add auto-fix label, verify agent comments with analysis after 3 attempts
• Verify deduplication: re-adding auto-fix label doesn't trigger if PR exists
• Trigger triage cron manually (workflow_dispatch), verify it only labels suitable issues
• Verify triage skips issues with comments, auto-fix-skip label, and cross-repo fixes

🤖 Generated with Claude Code