Docs: Add CONTRIBUTING.md with setup, development, and PR guidelines

[ShivamH1]

Problem

New contributors have no guidance on how to set up the development environment, run tests, or submit changes. The repository currently lacks:

• A CONTRIBUTING.md file
• Development setup instructions
• Code style conventions
• PR submission guidelines

Proposed Solution

Create a CONTRIBUTING.md at the project root covering:

1. Prerequisites — Node.js 22-26, npm
2. Getting Started — Fork, clone, npm install, npm run build
3. Development Workflow — Branch naming, making changes, npm run typecheck + npm test
4. Code Style — TypeScript strict mode, ESM modules, existing conventions
5. Testing — How to run all tests (npm test), individual tests (npm run test:*), smoke tests (npm run smoke:*)
6. Pull Request Process — What to include, how PRs are reviewed
7. Project Structure — Overview of apps/, libs/, scripts/, skills/

Why This Matters

• Lowers the barrier for first-time contributors
• Sets clear expectations for contributions
• Reduces review overhead by standardizing PR format

[ShivamH1]

@kushalpatil07 Please review this and let me know if anything is missing.