Update README: sync modes, Moonraker Spoolman setup, remove RFID auto-link docs

- Document direct vs Moonraker plugin sync modes
- Add step-by-step Moonraker [spoolman] config and trusted_clients note
- Replace RFID auto-link section (removed) with SSH serial number auto-link
- Add Fluidd panel section
- Update config.json example to include spoolman_mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>