Align setup instructions with the environment variables the code reads

[cubrift]

Problem

The setup examples use inconsistent environment variable names. README.md currently shows OPEN_API_KEY, while the runtime reads OPENAI_API_KEY in messages/ai/MessageAI.js. CONTRIBUTING.md also lists PHONE_NUMBER, although the current startup flow does not read it from the environment.

A new contributor who follows the README can end up with an apparently configured bot that cannot call OpenAI.

Expected behavior

The README and contributing guide should describe the variables that the current code actually requires and use the same names everywhere.

Acceptance criteria

• All setup snippets use OPENAI_API_KEY.
• GIPHY_API_KEY is documented consistently.
• The docs either remove PHONE_NUMBER or clearly explain and implement its use if it is intended to be required.
• The quick-start and contributor setup sections agree with each other.
• A fresh contributor can identify the required variables without inspecting the source code.

[cubrift]

Fixed by @lokeshsahu2804-korba