feat(ci): add feature agent workflow and rename labels to agent-fix/agent-feat

[peppescg]

Summary

• New feature agent (_feat-agent.yml + feat-on-label.yml): triggered by agent-feat label on Task/enhancement issues
  • Phase 1 (Opus): analyzes the issue, searches codebase, writes implementation-plan.md
  • Phase 2 (Sonnet): implements the feature following the plan, writes tests, runs checks
  • Hard gate: CI verifies all checks pass
  • Creates PR on success, comments on issue on failure
• Rename labels: auto-fix → agent-fix across bug-fix agent and triage cron

Feature Agent Flow

┌──────────────────────────────────────┐
│  TRIGGER (feat-on-label.yml)         │
│  if: "agent-feat" label              │
│  + "Task" or "enhancement" label     │
└─────────────────┬────────────────────┘
                  ▼
┌──────────────────────────────────────┐
│  FEATURE AGENT (45 min timeout)      │
│                                      │
│  GUARD: skip if PR exists or         │
│         agent already commented      │
│                                      │
│  PHASE 1 — Plan (Opus)              │
│  • Read feature request              │
│  • Search codebase for patterns      │
│  • Write implementation-plan.md      │
│  • No source modifications           │
│                                      │
│  PHASE 2 — Implement (Sonnet)       │
│  • Follow the plan                   │
│  • Write code + tests                │
│  • Suite + lint + types              │
│                                      │
│  HARD GATE: CI re-runs all checks    │
│                                      │
│  ✅ SUCCESS        ❌ FAILED          │
│  • feat/auto-N     • Comment with    │
│  • Open PR           plan            │
│  • agent-feat      • Remove label    │
│    label                             │
└──────────────────────────────────────┘

Test plan

• Create a Task issue with a small feature request, add agent-feat label, verify agent creates PR
• Verify label rename: add agent-fix to a Bug issue, verify bug-fix agent triggers
• Verify triage cron uses agent-fix label

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