Add GitHub and PostgreSQL MCP servers

[codegen-sh]

This PR adds two common MCP packages to the CLICP project as requested in CG-18914:

🚀 New MCP Servers Added

GitHub Server (@modelcontextprotocol/server-github)

• File Operations: Create/update files, get file contents
• Issue Management: Create issues, list/update issues, add comments
• Pull Request Management: Create PRs, list PRs, manage reviews
• Repository Operations: Search repositories, create repositories, fork repositories
• Search Capabilities: Search code, issues, and users

PostgreSQL Server (@modelcontextprotocol/server-postgres)

• Database Queries: Execute read-only SQL queries
• Schema Inspection: Access table schemas and metadata
• Safe Operations: All queries run in READ ONLY transactions

🔧 Configuration

Both servers support environment variable configuration:

• GitHub: Set GITHUB_PERSONAL_ACCESS_TOKEN for API access
• PostgreSQL: Set POSTGRES_CONNECTION_STRING (defaults to postgresql://localhost/postgres)

✅ Testing

• All existing tests pass
• Added comprehensive tests for new server integrations
• CLI commands work correctly:
  • clicp list shows all 4 servers
  • clicp github list shows GitHub tools
  • clicp postgres list shows PostgreSQL tools

📚 Documentation

• Updated README with new server documentation
• Added usage examples and configuration notes
• Updated feature list to include new servers

🎯 Implementation Details

• Follows existing MCPService patterns for consistency
• Maintains backward compatibility
• Uses official @modelcontextprotocol packages for reliability
• Proper error handling and validation

Resolves CG-18914

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