Docs/p4 documentation

[tejasprasad2008-afk]

P4-24 -> .env.example — Created with all configurable values documented: API key format, model options, debounce timings, session TTL, max suggestions/tabs/history, suggestion validation, UI timings, and
blocked domains with categories.

P4-25 — `.eslintrc.json` — Created with browser, es2022, webextensions, jest, and node environments. Rules for no-unused-vars (warn), no-console (allow warn/error), no-undef (error), prefer-const, eqeqeq,
curly. chrome global declared as readonly.

P4-26 — `babel.config.json` — Already existed with @babel/preset-env. No change needed — Babel is actively used by Jest's babel-jest transformer.

P4-27 — `CONTRIBUTING.md` — Completely rewritten with step-by-step instructions: prerequisites, clone, npm install, load unpacked extension in Chrome, get Groq API key, configure, verify it works, run
tests, lint/format. Also includes project structure map, contribution workflow, and good-first-contribution suggestions.

P4-28 — `CHANGELOG.md` — Created in Keep a Changelog format with [Unreleased] (documenting all recent changes across this session) and [1.0.0] (initial release features).

P4-29 — JSDoc annotations — Added to every public function and class across all source files:
 - config-manager.js — @typedef Config, all 9 public methods with @param/@returns
 - session-tracker.js — @typedef SessionData, IntentContext, all 7 methods
 - groq-service.js — @typedef Suggestion, SuggestionResponse, FieldMeta, GenerationContext, all 11 methods
 - context-collector.js — @typedef TabInfo, PageContext, FullContext, all 10 methods
 - form-detector.js — @typedef FormFieldMeta, FieldInput, all 4 public methods + 3 extraction helpers
 - browser-detector.js — detectOS(), detectBrowser()
 - language-detector.js — isSpokenLanguageField(), detectLanguages()
 - constants.js — Module doc with @module

All 52 tests still pass.