feat: Add GitHub MCP agent implementation with README and agent script

[zamalali]

Summary

This PR adds a new sample here contributing\samples\mcp_stdio_github_agent GitHub MCP Agent implementation to our ADK codebase. Key changes include:

• New agent.py

  • Loads GitHub Personal Access Token via .env or environment variable
  • Spawns the official ghcr.io/github/github-mcp-server Docker image over stdio
  • Instantiates an LlmAgent (gemini-2.0-flash) wired to the MCP server
  • Provides a simple REPL loop for interactive testing

• Prompt Definition

  • Guides the model to list/search repos, manage issues & PRs, and reuse context

• Module Export (__init__.py)

  • Exposes the github_agent for import into other components

I kept this very minimalistic and clean without adding any sort of complexity.
Please review and let me know if any adjustments are needed!

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