Allow public_repo scope for public repository contribution tools

[paulcakeface]

Describe the feature or problem you’d like to solve

Several public-repository write tools currently declare repo as their minimum OAuth scope. That prevents a token intentionally limited to public_repo from using those tools, even though GitHub's OAuth scope documentation defines public_repo as read/write access to public repositories.

A practical example is a public-only open-source contribution workflow. A token restricted to public_repo should be able to:

• fork a public repository
• create a branch in a public fork
• push files to that branch
• open a pull request
• create/update public issues
• comment on public issues or pull requests

The GitHub API already permits these public-repository operations with public_repo; requiring repo at the MCP metadata/filter layer asks for private-repository access unnecessarily.

Proposed solution

Use scopes.PublicRepo as the minimum declared scope for the existing public-capable contribution tools:

• fork_repository
• create_branch
• push_files
• create_pull_request
• issue_write
• add_issue_comment

NewTool already expands PublicRepo so repo remains an accepted parent scope. Existing private-repository workflows using repo therefore remain compatible, while public-only OAuth deployments can keep the narrower grant.

This would only change scope metadata/filtering; the GitHub API remains the authorization boundary and continues to reject operations the token or user cannot perform.

Example prompts or workflows (for tools/toolsets only)

• "Fork this public repository, make the documentation fix on a branch, and open a pull request."
• "Open an issue on this public repository with the reproduction steps."
• "Add this clarification as a comment on the public issue."

Additional context

I have a focused patch with regression coverage ready and can submit it against current main.

[paulcakeface]

Duplicate of #3136.