0.2.0: light backups and log trimming

What changed

Backups are now light. A full copy of the session tree is taken once every 24 hours (the newest 10 are kept). Every other run that writes anything copies only the files it is about to overwrite or delete, into backups/<timestamp>-changes/ (the newest 1000 are kept, and they never push out a full backup). Before this, every writing run made a full copy: measured on the author's Mac, that was about 624 copies a day of a ~182 MB tree, and the ring of 10 covered roughly three minutes.

A copy killed halfway never passes for a backup. Both kinds are written under a .partial- name and renamed into place only when complete.

The agent's logs stop growing. sync --trim-logs, which the background agent now passes, cuts a log that has passed 5 MB back to its newest 512 KB.

README: a comparison with claude-transplant, claude-account-switcher and the side-by-side launchers.

Upgrading

git pull
python3 bridge.py install-agent

On macOS the background agent runs a copy of the script, so run install-agent again to pick up the new version.

Thanks

Both changes follow the design in Zakaria Mehbi's fork.