Allow public_repo scope for public repository write tools #3136

Describe the feature or problem you'd like to solve

Several repository write tools currently declare scopes.Repo, so the OAuth scope filter hides them when the server is authenticated with a token limited to public_repo.

That forces an OAuth deployment which only needs to contribute to public repositories to request the broader repo scope, which also grants private-repository access.

The affected tools I have exercised successfully against public repositories with a public_repo token are:

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

The existing scope hierarchy already models repo as a parent of public_repo, so declaring scopes.PublicRepo for these tools would still accept a full repo token while also allowing least-privilege public-only OAuth deployments.

Proposed solution

Change the required scope for those six tools from scopes.Repo to scopes.PublicRepo and add focused tests asserting that each tool advertises:

• required scope: public_repo
• accepted scopes: public_repo, repo

No API handler or authorization bypass is needed; GitHub continues to enforce the token's actual repository permissions.

Example workflow

A public-only MCP deployment can request only public_repo, fork an open-source repository, create a branch, push a patch, open a pull request, and participate in issue discussion without gaining access to private repositories.

I have a small patch and regression coverage ready if this direction looks suitable.