fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Makes all 16 previously-failing tests pass (now 60 pass / 0 fail) by implementing missing functionality and fixing edge-case bugs across the utility library. No test files were modified and no dependencies were added.

Changes

• calculator — divide now throws Error("Division by zero") when the divisor is 0.
• string-utils — implemented truncate (truncates at a word boundary with "..." counting toward maxLength); fixed wordCount to split on /\s+/ so consecutive spaces don't inflate the count.
• task-manager — implemented remove (returns false for unknown id), update (merges changes, false for unknown id), and sortBy (non-mutating sort by createdAt, priority, or status).
• date-utils — fixed formatRelative off-by-one: uses Math.round on the hours→days conversion (e.g. 36h → "2 days ago").
• validator — isEmail now accepts subdomains and long TLDs; isUrl no longer rejects URLs without an explicit port.

Also removed two stale BUG: comments in date-utils.ts and validator.ts that described the now-fixed bugs.

Testing

bun test → 60 pass, 0 fail, 70 expect() calls.

Assumptions

• Kept fixes minimal and scoped strictly to what the tests assert.
• sortBy returns a new sorted array rather than mutating the internal task map.

Reviewed by the review subagent (verdict: approve).