Add support for custom agent to assign_copilot_to_issue

[snazy2000]

Summary

Updates the assign_copilot_to_issue tool to allow setting the custom_agent property, was previously always sending empty string

Why

Fixes #1392

What changed

• Added ability to select custom agent when assigning issues
• Added Tests

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• Assign issue X to copilot with custom agent Y

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)