Add update_pull_request_comment and delete_pull_request_comment tools

首先定位现有的 add_reply_to_pull_request_comment 工具及其 GitHub API 请求模式。使用所述的 REST 端点和参数添加匹配的更新和删除工具，然后验证每个工具都如描述的那样针对经过身份验证的用户的评论，并支持更新所请求的请求体。

Describe the feature or problem you'd like to solve

The MCP server provides add_reply_to_pull_request_comment for PR review comments, but there is no way to edit or delete an existing review comment. The only available mutation is adding a new reply underneath an existing comment.

When an agent needs to correct or retract a review comment it previously posted, the current workaround is to post a new reply — leaving the stale original visible and creating thread noise. The alternative is falling back to gh api with PATCH/DELETE on /repos/{owner}/{repo}/pulls/comments/{id}, bypassing the MCP server entirely.

Proposed solution

Add two new tools that map to the existing GitHub REST API endpoints:

update_pull_request_comment

• REST API: PATCH /repos/{owner}/{repo}/pulls/comments/{comment_id}
• Parameters: owner, repo, comment_id (number), body (string — new body text)
• Auth constraint: Can only update comments authored by the authenticated user
• Equivalent gh command: gh api repos/{owner}/{repo}/pulls/comments/{id} -X PATCH -f body="..."

delete_pull_request_comment

• REST API: DELETE /repos/{owner}/{repo}/pulls/comments/{comment_id}
• Parameters: owner, repo, comment_id (number)
• Auth constraint: Can only delete comments authored by the authenticated user
• Equivalent gh command: gh api repos/{owner}/{repo}/pulls/comments/{id} -X DELETE

Both endpoints are already documented in the GitHub REST API and require no new upstream features.

Additional context

This gap caused a real-world problem: an agent posted review replies that should have been edits to the original comments. The discussion_r{N} URL identifier ≠ the API comment id (a separate but related confusion — see #2235 for a different comment-reading gap), and without an edit tool the agent defaulted to the only mutation available — add_reply — producing thread noise instead of correcting the original.

Having edit/delete tools also enables agents to clean up their own mistakes programmatically, which is important for production reliability.