fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library. 60/60 tests now pass, none previously-passing were broken. Only source files were changed — no test files or dependencies were touched.

Changes

File	Fix
src/calculator.ts	divide() throws Error("Division by zero") when the divisor is 0 instead of returning Infinity.
src/string-utils.ts	Implemented truncate() — truncates at the last word boundary with "..." counting toward maxLength, returns the string unchanged when str.length <= maxLength. Made wordCount() robust to tabs/newlines/multiple spaces.
src/task-manager.ts	Implemented the missing/incomplete remove, update, and sortBy TaskManager methods.
src/date-utils.ts	formatRelative() off-by-one fixed by using Math.round semantics (e.g. 36h → "2 days ago").
src/validator.ts	Fixed isEmail regex and isUrl edge cases so the asserted inputs validate correctly.

Verification

• `bun test` → 60 pass, 0 fail.
• Independent review pass (verdict: COMMENT / no blockers).

Assumptions & notes

• Scope kept to exactly what the tests require.
• Non-blocking follow-ups flagged in review (pre-existing, untested edge cases — not regressions): `formatRelative` half-unit boundaries; `isEmail` consecutive-dot/IP-literal acceptance.