SessionManager._load() migrates sessions from the legacy directory
(~/.nanobot/sessions/) to the workspace path, but delete_session only
checked the workspace path. A user deleting a session could therefore
see its history come back the next time the session was loaded.
- delete_session now attempts to unlink both paths
- returns True if at least one file was removed
- added regression tests: legacy-only, both-paths, and no-revival