Recurring tasks: advanced schedules

[sjcotto]

Task.recurrence currently supports a fixed set (DAILY/WEEKDAYS/WEEKLY/BIWEEKLY/MONTHLY) handled in lib/recurrence.ts + lib/tasks.ts spawn-on-complete.

Scope

• Custom intervals ("every N days/weeks/months"), specific weekdays, monthly "nth weekday", and end conditions (until date / after N occurrences).
• Option to regenerate on schedule vs. on completion.

Acceptance

• "Every 2nd Tuesday, 5 times" spawns correctly and stops after 5.