Resolve conflicts from main's mount-prompt refactor against the safety
approval UI:
- update.go / vulnerabilities.go: keep the variadic cornerPrompt + cornerButton
needed for the three-button safety prompt while adopting main's mount changes
(Mount/Skip labels, mountPromptBounds + labelHitAt, direct answerMountConfirmation).
- Reconcile main's new runner-lifecycle tests (interrupt, teardown) with the
now-default guarded mode by running them with safety off, matching their intent.
Full Python (1168) and Go suites, ruff, and mypy strix/ pass on the merge.