agent start: a signal after worktree creation can leave the created worktree behind #672

Follow-up from #648 (merged d981d48).

tps agent start creates the agent's worktree before launch and removes it in the launchAttested finally (packages/cli/src/commands/agent.ts ~1349). The launcher's SIGINT/SIGTERM/SIGHUP handlers (~1100) remove the private config snapshot and re-raise the signal, which can end the process before that finally runs, so a signal after worktree creation can leave the created worktree behind.

Want: the signal path removes a worktree this launch created (and only one it created), or the next launch reconciles it; a test that signals the launcher after worktree creation and asserts the created worktree is gone (or reconciled) while a pre-existing worktree is untouched. Related: #668 (forward the signal to the released agent), #667 (stale snapshot sweep).

Refs #648