Stop posting retest boilerplate on maintainer issue reopens

The retest-comment workflow fired on 'reopened' as well as 'opened', and
canPostRetestComment gated on the issue author's association, not the
reopen actor's. Since non-collaborator reporters cannot reopen
maintainer-closed issues, a reopened event is in practice always a
deliberate maintainer decision, and the bot would post version-retest
boilerplate that contradicts the maintainer's own comment while planting
the needs-retest auto-close marker on an issue they chose to keep open
(seen on #1471).

Restrict the trigger and the guard to 'opened' and pin the reopened skip
with a test.

.github/scripts/issue-version-triage.test.cjs

@@ -145,6 +145,25 @@ test("postRetestComment comments once for older non-maintainer bug reports", asy
   );
 });
 
+test("postRetestComment skips reopened issues", async () => {
+  const { github, calls } = createGithub({ latestVersion: "6.0.1" });
+  const issue = {
+    number: 1471,
+    title: "Disk temperature at 0°C",
+    body: "## Feedback type\nBug / regression\n\n## Pulse version\n5.1.31\n",
+    labels: [],
+    author_association: "NONE",
+  };
+
+  await triage.postRetestComment({
+    github,
+    context: createContext({ action: "reopened", issue }),
+    core: createCore(),
+  });
+
+  assert.equal(calls.createComment.length, 0);
+});
+
 test("postRetestComment skips maintainer-authored issues", async () => {
   const { github, calls } = createGithub({ latestVersion: "6.0.1" });
   const issue = {