When worktree exists but session is missing, ensure_session was
incorrectly removing the worktree before recreating. This caused
issues when cmd_continue called ensure_worktree first (creating the
worktree) then ensure_session (which wrongly removed it).
The fix removes the block that removes worktree when session is missing.
If worktree exists, just create the session without touching the worktree.
Fixes#229
