feat(gitlab): Add MR closes_issues API endpoint and GitLabIssueRef type

Extends the GitLab client to fetch the list of issues that an MR will close
when merged, using the /projects/:id/merge_requests/:iid/closes_issues endpoint.

New type:
- GitLabIssueRef: Lightweight issue reference with id, iid, project_id, title,
  state, and web_url. Used for the closes_issues response which returns a list
  of issue summaries rather than full GitLabIssue objects.

New client method:
- fetch_mr_closes_issues(gitlab_project_id, iid): Returns Vec<GitLabIssueRef>
  for all issues that the MR's description/commits indicate will be closed.

This enables building the entity_references table from API data in addition to
parsing system notes, providing more reliable cross-reference discovery.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/mod.rs

@@ -1,5 +1,3 @@
-//! GitLab API client and types.
-
 pub mod client;
 pub mod transformers;
 pub mod types;
@@ -10,7 +8,7 @@ pub use transformers::{
     transform_discussion, transform_issue, transform_notes,
 };
 pub use types::{
-    GitLabAuthor, GitLabDiscussion, GitLabIssue, GitLabLabelEvent, GitLabLabelRef,
+    GitLabAuthor, GitLabDiscussion, GitLabIssue, GitLabIssueRef, GitLabLabelEvent, GitLabLabelRef,
     GitLabMergeRequestRef, GitLabMilestoneEvent, GitLabMilestoneRef, GitLabNote,
     GitLabNotePosition, GitLabProject, GitLabStateEvent, GitLabUser, GitLabVersion,
 };