feat: add heartbeat status updates during deployment

Send periodic "running" status messages while nixos-rebuild executes,
preventing the idle timeout from triggering before deployments complete.
This fixes false "Some deployments failed" warnings in MCP when builds
take longer than 30 seconds.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/messages/messages.go

@@ -35,6 +35,7 @@ const (
 	StatusAccepted  Status = "accepted"
 	StatusRejected  Status = "rejected"
 	StatusStarted   Status = "started"
+	StatusRunning   Status = "running"
 	StatusCompleted Status = "completed"
 	StatusFailed    Status = "failed"
 )