Announce a merged pull request's branch, not its pull ref, so its delivery group is found (hq issue 578)

The merge tool deletes the branch after merging; the forge then reads head.ref as refs/pull/<n>/head and only
head.label names the branch. pull.merged carried the pull ref, so the controller found no group for the merge,
cut mesh-catalog #251's walk at once and walked it before mesh-controller #271's, against the group's order.

modules/gitea/client.ts

@@ -49,6 +49,23 @@ export interface GiteaIssue {
   body?: string;
 }
 
+/**
+ * **The branch a pull request was opened from, also once it is deleted** (novox/hq issue 578). The forge says it as
+ * `head.ref` while the branch exists; once it is gone — the merge tool deletes it after the merge by default —
+ * `head.ref` reads `refs/pull/<n>/head`, and only `head.label` still names the branch. A delivery group is the pull
+ * requests sharing a head branch name (novox/hq ADR 0249), and the controller finds a merge's group, and joins the
+ * group's merges in one batch, by the head its `pull.merged` names (ADR 0276 decision 5, ADR 0306 decision 2). With
+ * the pull ref it found no group: on 2026-10-12 mesh-catalog #251's walk was cut at once and went before
+ * mesh-controller #271's, which the group's order put first.
+ */
+export function headBranchOf(head: { ref?: unknown; label?: unknown } | null | undefined): string | undefined {
+  const said = (v: unknown): string | undefined => (typeof v === "string" && v !== "" ? v : undefined);
+  const ref = said(head?.ref);
+  const label = said(head?.label);
+  if (ref === undefined || /^refs\/pull\/\d+\/head$/.test(ref)) return label ?? ref;
+  return ref;
+}
+
 /** A pull request, trimmed to the fields a reviewer or an event body needs. */
 export interface GiteaPull {
   number: number;
@@ -673,7 +690,7 @@ export class GiteaClient {
       merge_commit_sha: p.merge_commit_sha ?? undefined,
       merged_at: p.merged_at ?? undefined,
       user: p.user?.login,
-      head: p.head?.ref,
+      head: headBranchOf(p.head),
       head_sha: p.head?.sha ?? undefined,
       base: p.base?.ref,
       updated_at: p.updated_at ?? undefined,