docs: add JSDoc to all exported functions in src/ - #464

Summary

Adds JSDoc comments (description, @param for every parameter, @returns, and @throws where the function actually throws) to every exported function under src/. Comment-only change — no behaviour, signature, or export changes. Nothing outside src/ touched.

Files changed

• src/calculator.ts — add, subtract, multiply, divide (documents that divide returns Infinity/NaN on zero rather than throwing)
• src/string-utils.ts — capitalize, reverse, truncate (@throws — currently a "not implemented" stub), slugify, wordCount
• src/task-manager.ts — TaskManager class plus public methods add, get, list, complete, remove, update, sortBy (remove/update/sortBy get @throws — "not implemented" stubs)
• src/date-utils.ts — formatRelative, isSameDay (existing one-line summary reworded to mention local time), addDays
• src/validator.ts — isEmail, isUrl, isUsername, isAustralianPhone

Assumptions

• Public methods of the exported TaskManager class were treated as exported functions and documented.
• Exported types (Priority, Status, Task) are not functions and were left undocumented.
• Docs describe current behaviour, including known bugs/stubs flagged by existing BUG/TODO comments (left untouched).

Verification

• bun test: 44 pass / 16 fail — identical before and after this change (failures are pre-existing stubs/bugs).
• git diff confirmed comment-only; reviewed (no critical/high issues; two minor wording inaccuracies fixed and re-reviewed).