feat: add /ponytail default <mode> to persist the default (#329) (#541)

Claude Code had no way to change the persistent default from within a session:
plain /ponytail switches are session-scoped by design ('sticks until session
end'), and only env/config could set the default. So /ponytail off never
survived a restart, which is what #329 hit.

Add /ponytail default <mode> to the mode-tracker hook, matching pi's existing
command: it writes defaultMode to config (survives restart) and does NOT touch
the current session flag, keeping plain switches transient. review is rejected
as a default (#377); only off/lite/full/ultra are accepted. Test covers persist,
transient-switch-does-not-persist, and review-rejected.

Closes #329