Add custom_agent and model attributes to issue management

[TarcosS]

Describe the feature or problem you’d like to solve

Currently, GitHub MCP issue management does not provide a built-in way to define which custom agent or model is responsible for an issue.

With the introduction of the new GitHub Agents features, repositories can now use multiple custom agents with different roles and model configurations. However, there is no native issue-level attribute to specify which agent or model should handle a specific task.

This makes agent routing, orchestration, and model control difficult in multi-agent workflows.

Proposed solution

Add optional custom_agent and model attributes to issues so they can be explicitly assigned to a specific GitHub Agent and model.

This would enable proper agent orchestration, clearer ownership, and better integration with the new GitHub Agents ecosystem.

Additional context

This feature would align MCP issue management with GitHub’s new agent-based workflows and make MCP more suitable for agent-native development.