fix: make all tests pass and implement missing utilities - #475

Summary

• calculator.divide: throws Division by zero when divisor is 0
• string-utils: implement truncate (word-boundary, ... counts toward maxLength, never exceeds maxLength); wordCount splits on any whitespace run
• TaskManager: implement remove, update (explicit allowed-field assignment), sortBy (priority/createdAt/status)
• date-utils.formatRelative: round days instead of floor (symmetric for past/future)
• validator: isEmail accepts long TLDs; isUrl accepts ports
• Removed stale BUG/TODO comments

bun test: 60 pass, 0 fail (was 44/16). No test files modified, no deps added.

Assumptions

• divide(x, 0) throwing is the intended contract (per tests); this changes prior Infinity/NaN behaviour.
• update may clear description when explicitly passed undefined; title/priority are skipped when undefined.
• sortBy("status") order: pending → in_progress → completed (untested).
• truncate with maxLength < 3 hard-slices without ellipsis.