… cleanup

Users were seeing many stale sandbox branches in Source Control after work finished. Cleanup now forwards an active-only prune mode so clean agent worktrees are removed while dirty worktrees stay visible, and unmerged branch refs are preserved for safety.\n\nConstraint: Preserve unmerged branch refs while hiding stale clean worktrees from VS Code\nRejected: Delete all clean agent branches during cleanup | would risk dropping unmerged branch pointers\nConfidence: high\nScope-risk: moderate\nReversibility: clean\nDirective: Use --keep-clean-worktrees when operators intentionally need every clean worktree to stay mounted\nTested: npm test; node --check bin/multiagent-safety.js; npm pack --dry-run\nNot-tested: Live VS Code panel behavior against existing user workspace settings