garden/lock.go: Lock() and Unlock() toggle the locked flag on
existing tracked entries. Errors on untracked paths. Persists
to manifest.
cmd/sgard/lock.go: sgard lock <path>..., sgard unlock <path>...
6 tests: lock/unlock existing entry, persistence, error on untracked,
checkpoint behavior changes after lock, status changes between
drifted and modified after unlock.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
