Add update_pull_request_comment and delete_pull_request_comment tools

## 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.