Assign custom agent to issue via assign_copilot_to_issue

### Describe the feature or problem you’d like to solve

I want to assign a Copilot coding agent with a [custom agent](https://docs.github.com/en/enterprise-cloud@latest/copilot/concepts/agents/coding-agent/about-custom-agents) to an issue via assign_copilot_to_issue

### Proposed solution

Add a parameter to the tool that enables passing in the name of the custom agent to assign to the issue.

### Example prompts or workflows (for tools/toolsets only)

"Assign the Contoso Implementation Planner agent to Issue #32"

Start at the assign_copilot_to_issue tool entry point and trace how its arguments are defined and passed to the GitHub API. Add support for the custom-agent name described in the issue, then verify that assigning an issue can pass that value while existing calls remain valid.

Written by the indexing model from the issue text.