fix: make all tests pass - #481

Summary

• calculator.divide: throw Division by zero when b === 0
• string-utils: implement truncate (word-boundary, ellipsis counted in maxLength); wordCount handles repeated whitespace
• task-manager: implement remove, update, sortBy (priority / createdAt / status)
• date-utils.formatRelative: Math.round instead of Math.floor for days
• validator: isEmail accepts TLDs of 2+ letters; isUrl accepts ports

bun test: 60 pass, 0 fail (was 16 failing). tsc --noEmit clean. No test files or dependencies changed.

Assumptions / follow-ups (not changed, out of scope)

• formatRelative rounds future durations asymmetrically (Math.round(-1.5) === -1).
• update ignores description: undefined, so a description cannot be cleared.
• Several stale BUG:/TODO: comments remain in date-utils, string-utils, task-manager and validator.