Claude Code background subagents do not survive a session restart

After a Claude Code session restart (context re-read, MCP reconnect), background Agent-tool subagents from the earlier process are gone: ListAgents shows only peer sessions, and no completion notice arrives. Their worktrees can hold half-finished uncommitted edits and stale lock dirs. Recovery that worked: (1) check each worktree with git log and git diff to see how far it got; (2) check for leftover lock files; (3) respawn each agent with a brief naming the exact tip SHA, the uncommitted diff it must review and finish, and which test commit is expected to go from red to green. Have agents keep a status file updated after each commit, so a respawned agent can pick up from it. Also re-arm any Monitor watches, which are stopped too.