feat(014): implement reactive agent triggering engine
- Migration 015: extends agents with trigger config, adds reactive_runs table
- Reactor engine: decision chain (mode, depth, budget, cooldown, sequential)
- Reactor store: SQLite persistence for runs with RFC3339 timestamps
- Reactor poller: K8s Job status polling (15s interval)
- Failure notifier: system DM to owner on job failure
- REST API: /api/runs, /api/runs/:id, /api/runs/:id/retry, /api/agents/reactive
- Agent model: trigger_mode, cooldown, budget, depth, k8s_image, pending_work
- K8s runner: GetClientset() for poller
- All 28 test packages pass (8 new reactor tests)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
