fix: resolve failing tests and implement missing utilities

[stooit]

Summary

Fixes all 16 failing tests (now 60/60 pass, tsc --noEmit clean). No test files or dependencies changed.

• calculator: divide throws Error("Division by zero") when divisor is 0.
• string-utils: wordCount splits on /\s+/ after trim; implemented truncate (word-boundary cut, ellipsis counted toward maxLength).
• validator: isEmail accepts subdomains/long TLDs (254-char guard); isUrl no longer rejects URLs with ports (http/https allowlist retained).
• date-utils: formatRelative rounds days to nearest (36h → "2 days ago").
• task-manager: implemented remove, update (partial, only provided fields), sortBy (priority high→low, status, createdAt ascending).

Assumptions

• Behaviour defined by existing tests; where silent, chose conservative behaviour (e.g. update ignores undefined fields).
• Non-blocking: negative maxLength in truncate is not guarded (not required by tests).