fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across the utility library. The suite now reports 60 pass / 0 fail (was 44 pass / 16 fail). No test files were modified and no dependencies were added.

Changes (src only)

• calculator.ts — divide now throws on division by zero instead of returning Infinity (b === 0 also catches -0).
• string-utils.ts — wordCount collapses consecutive whitespace (/\s+/); implemented truncate to cut at a word boundary with the ellipsis counting toward maxLength.
• task-manager.ts — implemented remove, update, and sortBy. Priority sort orders high > medium > low; sort is stable so equal keys keep insertion order.
• date-utils.ts — formatRelative now uses Math.round(Math.abs(...)), fixing the off-by-one (36h → "2 days ago") symmetrically for past and future.
• validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts host:port (e.g. http://localhost:3000). Added a JSDoc note that isUrl is a format check, not an SSRF control.

Verification

• bun test → 60 pass, 0 fail
• tsc --noEmit → clean
• Reviewed via subagent: verdict COMMENT, no blocking issues.

Assumptions / scope notes

• Constrained to "fix only what the tests require" — non-blocking review nits (negative maxLength guard, tab/newline word boundaries in truncate, patch-clear semantics in update) were left out as they have no test coverage and no in-repo callers.
• sortBy("status") needed some ordering because "status" is in the type union; chose in_progress > pending > completed. No test constrains it.
• Out-of-scope changes proposed by the implementation agent (lockfile generation, package.json/.gitignore edits, docs) were reverted to honour the "no new dependencies / fix only what's required" constraint.