fix: resolve failing tests and implement missing functionality - #457

Summary

• calculator.divide: throws Division by zero when b === 0
• string-utils: implemented truncate (word-boundary, ellipsis counts toward maxLength, safe for maxLength < 3); wordCount splits on /\s+/
• task-manager: implemented remove (returns boolean), update (partial update, false for unknown id), sortBy (priority/status/createdAt)
• date-utils.formatRelative: days rounded symmetrically via Math.round(Math.abs(hours)/24) (fixes off-by-one)
• validator: isEmail accepts long TLDs; isUrl accepts URLs with ports; stale BUG comments removed

Verification

• bun test: 60 pass / 0 fail (was 44/16)
• bunx tsc --noEmit: clean
• Reviewed twice (initial + follow-up re-review), no critical/high issues

Assumptions

• No test files modified, no dependencies added.
• truncate with maxLength < 3 returns a truncated ellipsis (not covered by tests; chosen to honour the max-length contract).
• sortBy("createdAt") sorts oldest first; update ignores undefined fields.