feat(watch): add --polling flag for SMB/CIFS/NFS mounted volumes

[mhajder]

Summary

Add PollingObserver support to oikb watch for network-mounted volumes (SMB/CIFS/NFS) where native kernel filesystem events are not available.

Problem

oikb watch uses watchdog's native Observer which relies on kernel-level events (FSEvents on macOS, inotify on Linux). These events are not propagated over network filesystems, changes made on SMB/CIFS/NFS shares are silently missed, and watch never triggers a sync.

Solution

• New --polling flag switches to PollingObserver, which detects changes via periodic stat() calls
• New --polling-interval option (default: 5s) controls poll frequency
• Debounce auto-tunes to 3s in polling mode (vs 1s native) to handle SMB write bursts
• All defaults remain unchanged - no breaking changes for existing users