fix: resolve failing tests across utility modules

[stooit]

Fixes all 16 failing tests (60/60 passing, tsc clean). No test changes, no new deps.

• calculator: divide throws on zero divisor
• date-utils: formatRelative 36h boundary rounding
• string-utils: wordCount handles repeated whitespace; truncate implemented
• task-manager: added remove, update, sortBy (priority, createdAt)
• validator: isEmail accepts multi-label domains/long TLDs; isUrl accepts ports

Assumptions/notes:

• sortBy('status') orders pending -> in_progress -> completed (untested).
• formatRelative rounds the signed value, so exact .5-day ties differ past vs future (36h future -> 'in 1 day'). Only past is tested; follow-up: round the magnitude.
• isUrl checks syntax/scheme only; it is not an SSRF guard.