* fix: include archived tasks in completed tasks over time chart
Archived tasks without a session completed_at were excluded from the
Completed Tasks Over Time chart. Use COALESCE(session.completed_at,
task.archived_at) as the completion date so archived tasks fall back
to their archive timestamp.
* fix: address review feedback - isolate completion paths in test
- Remove archived_at from task-regular so it only counts via session
completed_at (CodeRabbit nitpick)
- Add task-last-step-no-dates to verify COALESCE IS NOT NULL filter
excludes tasks with neither date (Claude suggestion)
---------
Co-authored-by: Kandev Agent <agent@kandev.dev>
João Salavisa