fix: repair failing tests and implement missing utility functions

[stooit]

Summary

Fixes all 16 failing tests across the utility library and implements the missing functionality. Test suite went from 44 pass / 16 fail → 60 pass / 0 fail. Only source files in src/ were changed — no test files modified, no dependencies added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts — wordCount collapses consecutive whitespace; truncate implemented (word-boundary cut, ... ellipsis counting toward maxLength, returns unchanged when short enough).
• src/task-manager.ts — implemented remove(id), update(id, changes), and sortBy(field) (priority ordered high > medium > low).
• src/date-utils.ts — fixed off-by-one in formatRelative (uses Math.round, so 36h → "2 days ago").
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with ports (e.g. http://localhost:3000).
• Removed stale BUG: comments left over from the fixes.

Verification

• bun test → 60 pass / 0 fail / 70 expect() calls.
• Reviewed by the review subagent (verdict: COMMENT, no blockers).

Assumptions

• Scope limited to what the tests require, per task constraints. Untested edge-case behaviours flagged in review were intentionally left unchanged to avoid regressing passing tests or over-reaching scope.