feat: add comprehensive health check endpoints with component monitoring

- Implement /health and /ready endpoints in pkg/health
- Add dynamic health checking that executes checks on each request
- Introduce health check registration system for plugging in component checks
- Register basic checks for provider status, agent status in gateway
- Add utility functions for common health checks (memory, dependencies)
- Ensure endpoints are compliant with Kubernetes probe requirements

cmd/picoclaw/internal/gateway/helpers.go

@@ -203,6 +203,7 @@ func gatewayCmd(debug bool) error {
 	// since the original ctx is already canceled.
 	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer shutdownCancel()
+	channelManager.StopAll(shutdownCtx)
 	deviceService.Stop()
 	heartbeatService.Stop()
 	cronService.Stop()