feat: webhooks & Kubernetes Job runner for event-driven agents
- Webhook registration via MCP (register_webhook, list_webhooks, delete_webhook)
- HMAC-SHA256 payload signing, SSRF-safe HTTP client, loop detection (depth 5)
- 8-worker goroutine delivery pool with exponential backoff retry (1s/5s/30s)
- Dead letter queue with auto-purge, auto-disable after 50 consecutive failures
- Per-agent rate limiting (60 deliveries/min)
- K8s Job runner (register_k8s_handler, list_k8s_handlers, delete_k8s_handler)
- Auto-detect in-cluster via InClusterConfig, NoopRunner fallback
- REST API for webhook deliveries, dead letters, K8s job runs and logs
- Web UI: webhook management, K8s handler pages, dead letters view
- MultiDispatcher fan-out pattern for webhook + K8s event dispatch
- SQLite migration 009: webhooks, webhook_deliveries, k8s_handlers, k8s_job_runs
- 51 tests across 9 test packages, all passing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
