Recover canonical mono repository and diagnose recurring git repo loss

The /home/agent/agents/slava-agent/mono directory is not recognized as a git repository during heartbeat git worktree list, although git fetch appears to succeed. This is the third occurrence in ~10 days. Restore a usable canonical repository (re-clone or recover from backup/worktree), verify git worktree list works, add a heartbeat guard that reports critical status if mono/ is missing or not a repo, and investigate root cause (host migration, cleanup script, disk issue). No branch-based work can be accepted until this is fixed.