Document .env requirements for AI tooling in README

[tobiu]

Update README.md to explicitly mention the need for a .env file in the workspace root to enable AI tooling.

Requirements:

• The .env file must contain:
  • GEMINI_API_KEY: For the generative AI features.
  • GH_TOKEN: For the GitHub workflow MCP server.
• Mention that this file is already .gitignored by default for security.