Add CONTRIBUTING.md with macOS local development setup guide

[jeanduplessis]

Summary

• Adds a comprehensive CONTRIBUTING.md for macOS local development setup
• Covers prerequisites (Xcode CLI tools, Homebrew, Git LFS, Node 22 via nvm, pnpm, Docker, Vercel CLI, Stripe CLI)
• Documents project setup steps: clone, install, env vars, database, migrations, dev server
• Explains fake login and admin access conventions (@admin.example.com)
• Documents enterprise trial management in local dev (dev org creation, require_seats bypass, admin panel)
• Documents Cloudflare Workers and AI inference in local dev (port assignments, limitations, what works without workers)
• Includes common troubleshooting section

[kiloconnect]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

This PR adds a comprehensive CONTRIBUTING.md with macOS local development setup instructions, replaces the inline setup steps in README.md with a link to the new guide, and removes the old dev/macos dev-environment installation.txt.

I verified the documentation against the actual codebase:

• ✅ Package scripts (dev, test, validate, lint, format, etc.) match their descriptions
• ✅ Docker Compose config matches the database setup instructions
• ✅ .nvmrc and package.json engines confirm Node 22 requirement
• ✅ Pre-commit hook behavior matches the Git Workflow section
• ✅ lint-staged config matches the description (Prettier + typecheck)
• ✅ pnpm add -g vercel uses correct pnpm syntax (previously flagged, now fixed)
• ✅ pnpm validate description accurately reflects lint:changed and format:changed (previously flagged, now fixed)
• ✅ src/db/schema.ts path is valid
• ✅ pnpm script:run command exists
• ✅ api/dev/create-kilocode-org route exists
• ✅ No HTML <img> tags (markdown-only)

Files Reviewed (3 files)

• CONTRIBUTING.md (new) - 0 issues
• README.md - 0 issues
• dev/macos dev-environment installation.txt (deleted) - 0 issues