Add discussion comment write operations to discussions toolset

[gfortaine]

Describe the feature or problem you'd like to solve

The discussions toolset currently only supports read operations. There's no way for AI agents to participate in GitHub Discussions — posting comments, marking answers, or managing their own replies.

I ran into this while trying to contribute to a spec discussion (modelcontextprotocol#1772). My agent could read the thread but couldn't post. The add_issue_comment tool returned 404 because Discussions use GraphQL, not the REST Issues API.

Proposed solution

Add comment-level write operations to the discussions toolset:

Tool	GraphQL Mutation
add_discussion_comment	addDiscussionComment
update_discussion_comment	updateDiscussionComment
delete_discussion_comment	deleteDiscussionComment
mark_discussion_comment_as_answer	markDiscussionCommentAsAnswer

This mirrors the existing pattern for issues (add_issue_comment, update_issue_comment).

Example prompts or workflows

1. "Read the latest comments on discussion [docker] build arm64 #42 and post a summary of the key points"
2. "Find unanswered questions in the Q&A category and draft helpful responses"
3. "Add a comment to discussion https://github.com/github/github-mcp-server/issues/1771 #1772 with my implementation feedback"
4. "Mark the most upvoted comment as the answer on discussion add iologger for debugging purposes #15"
5. "Fix the typo in my last comment on discussion chore: Create docker-publish.yml #30"

Additional context

The GraphQL client (githubv4) is already used in pkg/github/discussions.go for read operations. Happy to submit a PR if this feature is accepted.