Add tools for sub-issue endpoint

[martinnormark]

Closes: #154

Note that I am calling the GitHub REST API, due to sub-issues not yet in go-github - however, it was merged in google/go-github#3580 so I'm happy to let this wait until they cut a new release and refactor.

Example use

I have tested the tools with a test repo here: https://github.com/martinnormark/gh-mcp-tests

Specifically I created 3 issues, where martinnormark/gh-mcp-tests#1 is the parent. You can look at the history there to see how sub-issues were added, removed, then added again.

It gracefully handled permission error due to token grants:

Fixed the token and asked to retry:

Asking to list sub-issues:

Removing a sub-issue from the parent, not mentioning the parent:

Adding it back again:

Asking to re-order:

[Copilot]

Pull Request Overview

This PR adds new tooling endpoints for handling sub-issues within GitHub issues by extending the current server toolset. The changes include the introduction of a read tool for listing sub-issues and write tools for adding, removing, and reprioritizing sub-issues.

Comments suppressed due to low confidence (2)

pkg/github/tools.go:49

• [nitpick] Consider adding a brief comment explaining the intended usage of the ListSubIssues tool, similar to existing documentation for other endpoints.

toolsets.NewServerTool(ListSubIssues(getClient, t))

pkg/github/tools.go:56

• [nitpick] Consider adding inline comments for the new sub-issue write tools to clearly describe their behavior and expected input, ensuring consistency with the rest of the codebase.

toolsets.NewServerTool(AddSubIssue(getClient, t))