fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Makes the full bun test suite pass (60/60, up from 44 pass / 16 fail) by implementing missing functions and fixing edge-case bugs across five source files. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts — wordCount collapses consecutive whitespace; truncate implemented with word-boundary truncation + ellipsis, guarding against leading-whitespace content loss and never exceeding maxLength.
• src/task-manager.ts — implemented remove, update, and sortBy (priority high>medium>low, status ordering, createdAt oldest-first). sortBy has a throwing default so an unknown field fails at the boundary rather than returning undefined.
• src/date-utils.ts — formatRelative uses Math.round so 36h reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs and subdomains; isUrl accepts URLs with a port.

Verification

• bun test: 60 pass / 0 fail (70 expect() calls)
• tsc --noEmit: clean (exit 0)
• Only the 5 source files changed; no test files touched.

Review notes / assumptions

• Reviewed via the review subagent; three uncovered edge cases fixed in follow-up (truncate whitespace loss, sortBy default, truncate never exceeds maxLength).
• update(id, { description: undefined }) is intentionally a no-op; explicit "" still clears.
• Future work: isEmail is a pragmatic regex, not RFC 5322-compliant — use a confirmation-link round-trip as the trust boundary if gating account creation.

🤖 Generated autonomously.