First scratch workspace creation on an install now emits a one-shot
warning log + a 'tip_scratch_workspace' event on the task. Sentinel
file at ~/.hermes/kanban/.scratch_tip_shown silences subsequent
creations across the whole install.
Behavior unchanged — scratch is still ephemeral by design. This just
makes the design visible to new users (reported in user community:
'progress files vanished, no warning anywhere').
Docs (en + ko) updated to spell out 'Deleted when the task completes'
on the scratch bullet and 'Preserved on completion' on worktree/dir.