fix: resolve failing tests and implement missing utilities

[stooit]

Summary

• calculator: divide throws on zero divisor.
• string-utils: wordCount splits on any whitespace run; implemented truncate (word-boundary, ellipsis counts toward maxLength).
• task-manager: implemented remove, update (partial, !== undefined guards), sortBy (returns a copy; priority high→low, createdAt oldest first).
• validator: isEmail allows subdomains and TLDs of 2+ chars (reviewed: no ReDoS); isUrl accepts URLs with ports (http/https only).
• date-utils: formatRelative rounds day buckets (36h → "2 days ago"), symmetric for future dates.

Verification

• bun test: 60 pass / 0 fail (was 44/16)
• bunx tsc --noEmit: clean
• Test files unchanged; no new dependencies.

Assumptions

• sortBy('status') has no test coverage; chose order in_progress → pending → completed.
• divide throwing is a behaviour change for callers relying on Infinity; required by tests.