fix: implement missing utils functions and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests in the utility library — 60/60 now pass (was 44/60). No test files modified, no dependencies added; scope limited to what the tests require.

Fixes by file

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 (also covers -0), instead of silently returning Infinity/NaN.
• src/string-utils.ts
  • wordCount splits on /\s+/ against the trimmed string, correctly handling consecutive spaces, tabs, and newlines.
  • truncate implemented: returns unchanged when str.length <= maxLength; otherwise truncates at a word boundary with "..." counted toward maxLength.
• src/task-manager.ts — implemented remove (returns false for unknown id), update (partial update via !== undefined checks; returns false for unknown id), and sortBy (priority high>medium>low, createdAt oldest first, status in workflow order).
• src/date-utils.ts — formatRelative day bucket uses Math.round(Math.abs(diffHours) / 24), fixing the off-by-one at exactly 36 hours (now "2 days ago") while keeping past/future symmetric.
• src/validator.ts — isEmail allows TLDs of length >= 2 ({2,}); isUrl accepts URLs with an explicit port while still rejecting non-http(s) protocols.
• docs — removed stale BUG: comments in date-utils.ts and validator.ts that described the now-fixed defects.

Testing

bun test -> 60 pass, 0 fail. tsc --noEmit clean.

Assumptions / notes

• Reviewed but intentionally not changed (outside the tested contract, per "fix only what the tests require"): truncate with negative/NaN maxLength; truncate when the budget window is all whitespace; update clearing an optional field via explicit undefined. These are unguarded edge cases in newly-written code with no prior contract; flagging for a future hardening pass rather than expanding scope here.
• sortBy("createdAt") relies on stable sort + insertion order for same-millisecond ties (correct, but the test doesn't genuinely exercise timestamp comparison).

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