fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across 5 source files. All 60 tests now pass and tsc --noEmit is clean under strict: true. No test files modified, no dependencies added.

Changes

• calculator.ts — divide now throws on a zero divisor (was returning Infinity); guard catches both 0 and -0.
• string-utils.ts — wordCount splits on /\s+/ against the trimmed string so runs of whitespace collapse to one delimiter; implemented truncate (word-boundary cut with "..." counting toward maxLength, unchanged when it already fits).
• task-manager.ts — implemented remove, update, and sortBy (priority high>medium>low, createdAt oldest first).
• date-utils.ts — formatRelative uses Math.round instead of Math.floor, so 36h rounds to "2 days ago".
• validator.ts — isEmail TLD quantifier relaxed {2,4}→{2,} to accept long TLDs (e.g. .museum); isUrl no longer rejects URLs with ports.

Verification

• bun test: 60 pass / 0 fail
• tsc --noEmit: clean
• Reviewed: fixes confirmed correctly scoped, no regression against the 44 previously-passing tests (verified empirically against HEAD across adversarial inputs).

Assumptions / notes (not pinned by tests)

• update clears description when passed {description: undefined} (uses "description" in changes); title/priority use !== undefined to respect their non-optional types.
• sortBy("status") has no test coverage — chose workflow order pending → in_progress → completed.
• sortBy("createdAt") ties rely on stable sort + Map insertion order (ES2019 guarantees stability).
• Reviewer flagged a pre-existing edge case: truncate with a negative/NaN maxLength is unguarded. No caller passes negative values and it is untested, so left out of scope per "fix only what the tests require".