Add `plit logs` command to tail/view daemon logs

[theuseless-ai]

Summary

Now that plit start runs as a background daemon (logs redirected to ~/.local/share/plit/plit.log), users need a way to view logs without manually finding the file.

Proposed behavior

# Tail logs (like docker logs -f)
plit logs

# Show last N lines
plit logs --tail 100

# Follow mode (default)
plit logs --follow

Implementation notes

• Log file path: ~/.local/share/plit/plit.log (from config::log_path())
• Basic implementation: read + tail the log file, optionally follow with notify or polling
• Should check if plit is running (PID file) and warn if not
• --follow can use tokio::fs + polling or the notify crate for file watching