pull_request_read drops merge_commit_sha

從 pkg/github/pullrequests.go 開始，閱讀 GetPullRequest、convertToMinimalPullRequest 和 MinimalPullRequest。新增保留合併提交 SHA 的支援，然後驗證對於已合併的 pull request，使用 method get 的 pull_request_read 會公開 merge_commit_sha。

Describe the bug

pull_request_read with method: get does not return the pull request's merge_commit_sha, so there is no way to get from a merged pull request to the commit the merge produced without a second call.

In the source, GetPullRequest fetches the full pull request and then returns convertToMinimalPullRequest(pr). MinimalPullRequest carries merged, merged_at, merged_by, head and base, but has no merge_commit_sha field, so the value is dropped during conversion. merge_commit does not appear anywhere in pkg/github/pullrequests.go.

Affected version

Unknown. Observed through a hosted GitHub MCP integration on 2026-09-06, where the server version is not exposed to me. The code references above are from the repository's default branch on that date and may not match the build that was serving the request.

Steps to reproduce the behavior

1. Call pull_request_read with method: get for a merged pull request.
2. Inspect the returned object.

Expected vs actual behavior

Expected: merge_commit_sha is present for a merged pull request, as the REST API returns it.

Actual: the field is absent. Callers fall back to listing commits and matching on merge time or on the pull request number in the commit message, both of which are unreliable on an active branch.

Logs

None captured beyond the returned object.