fix: make pr merge idempotent for merged prs

scripts/gh-cli-pr-contract-test.ts

@@ -561,6 +561,15 @@ export async function runGhCliPrContract(): Promise<JsonRecord> {
     assertCondition(mergeRequest !== undefined, "pr merge should call GitHub REST merge endpoint", mock.requests);
     const mergePayload = JSON.parse(mergeRequest?.body ?? "{}") as JsonRecord;
     assertCondition(mergePayload.merge_method === "squash", "pr merge should pass selected merge method", mergePayload);
+    const mergeRequestCount = mock.requests.filter((request) => request.method === "PUT" && request.url === "/repos/pikasTech/unidesk/pulls/42/merge").length;
+    const alreadyMerged = await runCli(["gh", "pr", "merge", "43", "--repo", "pikasTech/unidesk", "--squash"], env);
+    assertCondition(alreadyMerged.status === 0, "pr merge should treat an already merged PR as idempotent success", alreadyMerged.json ?? { stdout: alreadyMerged.stdout });
+    const alreadyMergedData = dataOf(alreadyMerged.json ?? {});
+    assertCondition(alreadyMergedData.alreadyMerged === true, "already merged PR response should expose alreadyMerged=true", alreadyMergedData);
+    const alreadyMergedPullRequest = alreadyMergedData.pullRequest as JsonRecord | undefined;
+    assertCondition(alreadyMergedPullRequest?.merged === true, "already merged PR response should expose merged pullRequest", alreadyMergedData);
+    const mergeRequestCountAfterAlreadyMerged = mock.requests.filter((request) => request.method === "PUT" && request.url === "/repos/pikasTech/unidesk/pulls/42/merge").length;
+    assertCondition(mergeRequestCountAfterAlreadyMerged === mergeRequestCount, "already merged PR should not call REST merge endpoint again", mock.requests);
 
     const preflight = await runBun([
       "scripts/code-queue-pr-preflight-example.ts",

scripts/src/gh.ts

@@ -4900,6 +4900,19 @@ async function prMerge(repo: string, token: string, number: number, options: Git
   const pr = await githubRequest<GitHubPullRequest>(token, "GET", `/repos/${owner}/${name}/pulls/${number}`);
   if (isGitHubError(pr)) return commandError("pr merge", repo, pr, { number, phase: "fetch-pr" });
   const summary = prSummary(pr);
+  if (summary.merged === true) {
+    return {
+      ok: true,
+      command: "pr merge",
+      repo,
+      number,
+      method: options.mergeMethod,
+      alreadyMerged: true,
+      pullRequest: summary,
+      branchDeletion: { attempted: false, skippedReason: "already-merged" },
+      rest: true,
+    };
+  }
   const metadata = await prGraphqlMetadata(repo, token, number);
   if (isGitHubError(metadata)) return commandError("pr merge", repo, metadata, { number, phase: "fetch-pr-closeout-metadata", pullRequest: summary });
   const statusChecks = statusRollupSummary(repo, number, metadata.statusCheckRollup, false);