agent start: don't print manual-removal advice for a worktree the launcher's retry already removed #686

Found while building #685 (#676's launcher-level cleanup tests).

When setup's own removal of the worktree it created fails and the launcher's retry then removes it, tps agent start still prints setup's line — worktree cleanup failed for <path>: ...; remove it manually with: git -C '<base>' worktree remove --force '<path>' — before reporting the original error. By the time the launch exits, the worktree is gone, so the printed advice asks the user to remove something that no longer exists.

What the fix does: the manual-removal advice is printed only for a worktree that is still there when the launch exits. If the retry removes it, the output says the retry cleaned up (or says nothing about cleanup) and keeps the original error. Test: the existing "setup's own removal fails and the launcher's retry removes the worktree" case in packages/cli/test/sandboxed-launch-516.test.ts asserts the advice line is absent.