Remove a worktree the launch added when the launch fails before recording it #660

Found in the review of #648 (sandboxed launch).

When a selected-runtime launch adds a worktree but then fails before its git directories resolve (for example, onStart throws after git worktree add), the launch should remove the worktree it created. Today the parent records the worktree only after onStart returns, so a throw leaves it behind.

Acceptance: a test where git worktree add succeeds and the next step fails; afterwards the worktree and its branch are gone and git worktree list shows no stale entry.