a03c0eeb — on-demand real disk cleanup for this project's worktrees.

Reclaims any active_worktrees row whose owning sprint item/session has already reached a terminal state but whose directory was never actually removed from disk (see worktree_cleanup.sweep_stale_worktrees). This is the same pass the server runs periodically; exposed here so a post-integration trigger (e.g. the Stop-hook sprint guard, once a session's pending sprint items hit 0) doesn't have to wait for the next periodic tick. Self-hosted only — hosted (multi-tenant) mode has no filesystem access to the caller's worktrees and returns a no-op summary.