feat(client): add rich domain classes, helpers, and refactor GitLabClient internals

ts/gitlab.classes.branch.ts

@@ -0,0 +1,18 @@
+import type { IGitLabBranch } from './gitlab.interfaces.js';
+
+export class GitLabBranch {
+  public readonly name: string;
+  public readonly commitSha: string;
+
+  constructor(raw: IGitLabBranch) {
+    this.name = raw.name || '';
+    this.commitSha = raw.commit?.id || '';
+  }
+
+  toJSON(): IGitLabBranch {
+    return {
+      name: this.name,
+      commit: { id: this.commitSha },
+    };
+  }
+}