fix: resolve all failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests across 5 utility modules. The full suite now passes: 60 pass, 0 fail.

Changes

• calculator — divide now throws on division by zero instead of returning Infinity.
• string-utils — implemented truncate (returns unchanged within limit, respects the ellipsis budget for short maxLength, trims to a word boundary) and fixed wordCount to handle consecutive/leading/trailing whitespace.
• task-manager — implemented the missing/incomplete remove, update, and sortBy (priority high→medium→low, and createdAt oldest-first) methods.
• date-utils — fixed formatRelative off-by-one (rounds instead of flooring, so 36 hours reads "1 day ago").
• validator — fixed isEmail (long TLDs) and isUrl (URLs with explicit ports).
• Cleaned up stale BUG: comments that described the now-fixed behaviour.

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls.
• No test files modified, no new dependencies added — only src/ implementation changed.
• Reviewed by the review subagent: no blocking issues.

Assumptions

• Division by zero should throw (matches the test expectation) rather than return a sentinel value.
• truncate counts the ellipsis toward maxLength and prefers a word boundary, per the test cases.