Adds a distinct two-tone attention sound (880->660Hz) for approval and
clarify prompts so they are not confused with the existing completion sound,
plus sidebar attention badges + colored rails driven by `attention` metadata
on /api/sessions. Includes lock-safety note: the in-lock
publish_session_list_changed() calls in clarify.py are safe because publish()
only takes the leaf _SESSION_EVENTS_LOCK and never re-acquires clarify._lock
(verified by Opus advisor review).
Co-authored-by: ai-ag2026 <261867348+ai-ag2026@users.noreply.github.com>
