feat: shipnode logs — follow mode, multi-source, grep

[devalade]

What to build

Extend shipnode logs from a one-shot PM2 tail into a proper live-debugging tool that covers every log source shipnode already sets up on the server.

New flags:

• -f, --follow — stream logs continuously until Ctrl-C
• --source pm2|caddy|system|all — pick the log source (default: pm2, current behavior); all multiplexes them into a single stream with a per-line prefix like [pm2] / [caddy] / [system]
• --grep <pattern> — server-side filter (piped through grep on the remote so we don't drag the full stream over SSH)
• --lines n — keeps working, backfills the last n lines before entering follow mode

Sources map to concrete remote commands:

• pm2 → pm2 logs <app> --lines n [--raw]
• caddy → tail Caddy access + error logs (paths taken from the eject template so they stay in sync)
• system → journalctl -u <unit> -n n [-f]
• all → run each subcommand and interleave stdout with a source prefix

Acceptance criteria

• shipnode logs -f streams new lines until interrupted, then exits cleanly (SSH channel closed, no orphan remote process)
• shipnode logs --source caddy returns non-empty output on a server that has been serving traffic
• shipnode logs --source all -f shows interleaved lines with a [source] prefix
• shipnode logs --grep 500 filters on the remote (verify with set -x that grep runs there, not locally)
• --lines n still works, and combines with -f (backfill then follow)
• Ctrl-C leaves no zombie tail/pm2 logs/journalctl -f process on the server

Blocked by

None - can start immediately