feat(trail): show a spinner during tune's gather and agent run

The two opaque waits in tune — gathering repository signal (gh, checkpoint listing, trails API) and the headless agent run — printed a static line and then sat silent. Wrap both in the shared startSpinner helper so there's live "still working" feedback on a TTY, degrading to a "✓ <step>" line when output isn't a terminal (CI, pipes).