fix(utils): make all failing tests pass

[stooit]

Summary

Fixes all 16 failing tests in the utility library. Test suite now: 60 pass / 0 fail (bun test). No test files modified, no dependencies added — scope kept to exactly what the tests require.

Changes

File	Fix
src/calculator.ts	divide(x, 0) now throws instead of returning Infinity (also catches -0).
src/string-utils.ts	Implemented truncate (word-boundary cut, ellipsis counts toward maxLength, handles strings shorter than the ellipsis); wordCount now collapses consecutive whitespace via split(/\s+/).
src/task-manager.ts	Implemented remove (returns true/false), update (explicit field allowlist, false on unknown id), and sortBy (priority high<medium<low, createdAt oldest-first; sorts a copy so the internal Map order is preserved).
src/date-utils.ts	Fixed day-bucket off-by-one: Math.round(Math.abs(diffHours)/24) so 36h → "2 days ago", symmetric for past/future.
src/validator.ts	isEmail accepts subdomains and long TLDs (e.g. .museum) while still rejecting empty labels; isUrl accepts URLs with ports (http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail
• bunx tsc --noEmit → clean
• Independent code review: Approve, no blockers.

Assumptions & notes

• Scope kept to the tests only, per task constraint. Review surfaced non-blocking hardening opportunities beyond the test requirements, deliberately not actioned here:
  • truncate with a negative maxLength (no test covers it).
  • TaskManager.update/sortBy do no runtime value validation of caller-supplied priority — fine for the typed API, but a follow-up if reached from untrusted input.
  • isEmail/isUrl are syntax predicates, not security gates (no SSRF/length hardening added).
• sortBy("status") ordering (in_progress < pending < completed) is untested; chosen as a sensible "active work first" default.