fix: make all tests pass - #470

Summary

• calculator.divide: throw on division by zero
• date-utils.formatRelative: round days (symmetric for past/future) instead of floor
• string-utils: implement truncate (word boundary, ellipsis counts toward maxLength, guard for maxLength < 3); fix wordCount for repeated whitespace
• validator: isEmail accepts TLDs of 2+ letters; isUrl accepts ports
• task-manager: implement remove, update (whitelisted fields only), sortBy (priority/status/createdAt)
• Removed stale BUG/TODO comments

Assumptions

• sortBy("status") order is pending → in_progress → completed (untested).
• update({ description: undefined }) clears the description; undefined title/priority are ignored.

Testing

bun test: 60 pass, 0 fail (was 16 failing). No test files modified, no dependencies added.