Bug: agit cleanup does not prune stale worktrees detected by agit status #3

Description

agit status correctly detects stale worktrees (where the directory has been removed) and displays:

NOTICE: 2 worktree(s) marked stale (directory removed)

However, running agit cleanup immediately after reports "Nothing to clean up" — it does not remove the stale DB entries.

Steps to Reproduce

1. agit spawn myrepo --agent test --branch feat/test
2. rm -rf /path/to/repo/.worktrees/agit-<id>
3. agit status → shows "N worktree(s) marked stale (directory removed)" ✅
4. agit cleanup → "Nothing to clean up" ❌

Expected Behavior

agit cleanup should prune stale worktree entries from the database when their directories no longer exist on disk.

Environment

• agit version: 0.1.1
• OS: Linux (Ubuntu 22.04)
• Arch: amd64