Adds `buildAlertsTreeNodes` to sidebar-utils and renders an Alerts section
between Applications and Starred in LayoutShell. The section uses an
accordion pattern — entering `/alerts/*` collapses apps/admin/starred and
restores their state on leave.
gitnexus_impact(LayoutContent, upstream) = LOW (0 direct callers; rendered
only by LayoutShell's provider wrapper).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
