Add custom_instructions parameter to assign_copilot_to_issue tool

[Copilot]

Summary

Adds optional custom_instructions parameter to assign_copilot_to_issue tool, enabling users to provide additional context beyond the issue body when assigning Copilot agents.

Why

When assigning Copilot to issues, additional context like coding standards, architectural constraints, or testing requirements cannot be captured in the issue body. This parameter preserves that context by passing it directly to the agent.

What changed

• Added custom_instructions string parameter (optional) to tool schema
• Updated parameter parsing and AgentAssignmentInput to conditionally include custom instructions
• Added test case verifying custom instructions pass through GraphQL mutation correctly
• Updated toolsnaps and auto-generated documentation

MCP impact

• Tool schema or behavior changed
  • Added optional parameter to existing tool. Backwards compatible.

Prompts tested (tool changes only)

• "Assign copilot to issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 with instructions to follow PEP 8 and include comprehensive docstrings"
• "Assign copilot to issue Bump go-github to v72.0.0 #456 in owner/repo, use pytest for testing and maintain Python 3.8+ compatibility"

Security / limits

• No security or limits impact
  • Parameter value is passed directly to GitHub's agent assignment API with no additional processing

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

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

Docs

• Updated (README / docs / examples)
  • Auto-generated via ./script/generate-docs

Example usage

{
  "name": "assign_copilot_to_issue",
  "arguments": {
    "owner": "github",
    "repo": "my-repo",
    "issue_number": 123,
    "custom_instructions": "Follow PEP 8. Use pytest. Maintain Python 3.8+ compatibility."
  }
}

Original prompt

Assign copilot to issue tool should support option custom instructions param to guide agent beyond the issue body, as additional context is lost otherwise.

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