Vercel build was failing with TS18047 — `sprint.completed_at` is
possibly `null`. The earlier `.filter(s => s.completed_at != null)`
runtime-filtered the nulls out but did NOT narrow the element type;
TypeScript still saw `Date | null` on the result.
Add a user-defined type guard `(s): s is SprintWithCompletedAt =>` so
the narrowed array carries `completed_at: Date`. No runtime change.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Aggregates task.status=DONE counts across last N completed sprints
(default 5), filtered by productAccessFilter and returned in
chronological order for x-axis rendering.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
