fix: resolve failing tests and implement missing utilities - #460

Summary

• calculator.divide: throws Error("Division by zero") when b === 0.
• date-utils.formatRelative: days now rounded (Math.round(|hours|/24)) instead of floored; fixes off-by-one (36h → "2 days ago").
• string-utils: implemented truncate (word-boundary, ... counts toward maxLength, hard-cut fallback, tiny maxLength handled); wordCount splits on /\s+/ after trim.
• task-manager: implemented remove, update (whitelisted fields only), sortBy (priority, status, createdAt).
• validator: isEmail accepts TLDs of 2+ chars; isUrl accepts URLs with ports.
• Stale BUG/TODO comments updated.

Verification

bun test: 60 pass / 0 fail (previously 16 failing). tsc --noEmit clean. Reviewed twice by review agent; no blockers.

Assumptions

• Dividing by zero should throw (per tests) rather than return Infinity.
• TaskManager.update ignores undefined values for all fields, so description can't be cleared via undefined; easy to change if clearing is desired.
• No test files modified, no dependencies added.