ci(pr-checks): conventional-commit title check, unmergeable-PR flagging, pin actions by SHA (#3336)

* ci(pr-checks): add Conventional Commits PR-title check, pin actions by SHA

Add a check-title job that fails the PR when the title is not Conventional
Commits format (type(scope): summary), via an inline github-script regex.
Pin the workflow's actions to their latest release commit SHAs:
actions/checkout v6.0.3 and actions/github-script v9.0.0.

* ci(pr-checks): flag unmergeable PRs in the PR-checks workflow

Add a check-mergeable job to the (renamed) PR checks workflow: on PR events,
poll the PR's mergeable state and, when it conflicts with the base, remove
'ready for review', add a red 'merge conflict' label (auto-created), and
comment; clear the label once mergeable again. Single-PR, no push trigger.
Add ready_for_review to the trigger types.

* ci(pr-checks): drop the comment from check-mergeable, label swap only

* ci(pr-checks): least-privilege workflow permissions

contents:read for base-ref checkout, pull-requests:read for pulls.get
mergeability, issues:write for label + comment management. Drops the
unused pull-requests:write (labels and PR comments go through the issues
API).

.github/workflows/pr-description-check.yml

@@ -1,28 +1,103 @@
-name: ci / PR description check
+name: ci / PR checks
 
 on:
   pull_request_target:
-    types: [opened, edited, synchronize, reopened]
+    types: [opened, edited, synchronize, reopened, ready_for_review]
 
 # pull_request_target runs in the base-repo context (has secrets).
 # The checkout below pins to the base branch so no fork code is executed.
 # The script only reads context.payload and calls the GitHub API.
+# Least privilege: contents:read for the base-ref checkout, pull-requests:read
+# for pulls.get (mergeability), issues:write for label + comment management
+# (PR labels and comments both go through the issues API).
 permissions:
+  contents: read
+  pull-requests: read
   issues: write
-  pull-requests: write
 
 jobs:
   check-description:
     name: Check PR description
     runs-on: ubuntu-latest
-    # Skip bots — they open PRs programmatically and have their own process.
+    # Skip bots: they open PRs programmatically and have their own process.
     if: github.event.pull_request.user.type != 'Bot'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10  # v6.0.3
         with:
           ref: ${{ github.base_ref }}
           sparse-checkout: .github/scripts
 
-      - uses: actions/github-script@v7
+      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3  # v9.0.0
         with:
           script: return require('./.github/scripts/check-pr-description.js')({github, context, core})
+
+  check-title:
+    name: Check PR title (Conventional Commits)
+    runs-on: ubuntu-latest
+    # Skip bots: they open PRs programmatically and have their own process.
+    if: github.event.pull_request.user.type != 'Bot'
+    steps:
+      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3  # v9.0.0
+        with:
+          script: |
+            const title = context.payload.pull_request.title || "";
+            // Conventional Commits: type(optional-scope)(optional !): summary
+            const re = /^(feat|fix|docs|style|refactor|perf|test|build|ci|chore|revert)(\([\w .\/-]+\))?!?: .+/;
+            if (!re.test(title)) {
+              core.setFailed(
+                `PR title is not in Conventional Commits format:\n  "${title}"\n\n` +
+                `Expected: type(scope): summary\n` +
+                `Example:  fix(search): handle empty query\n` +
+                `Types: feat, fix, docs, style, refactor, perf, test, build, ci, chore, revert.`
+              );
+            } else {
+              core.info(`PR title OK: ${title}`);
+            }
+
+  check-mergeable:
+    name: Flag unmergeable PRs
+    runs-on: ubuntu-latest
+    # Skip bots: they open PRs programmatically and have their own process.
+    if: github.event.pull_request.user.type != 'Bot'
+    steps:
+      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3  # v9.0.0
+        with:
+          script: |
+            const repo = { owner: context.repo.owner, repo: context.repo.repo };
+            const number = context.payload.pull_request.number;
+            const READY = "ready for review";
+            const CONFLICT = "merge conflict";
+
+            // Ensure the conflict label exists (red). Ignore if already present.
+            try {
+              await github.rest.issues.getLabel({ ...repo, name: CONFLICT });
+            } catch {
+              await github.rest.issues.createLabel({
+                ...repo, name: CONFLICT, color: "B60205",
+                description: "Conflicts with the base branch; needs a rebase before review.",
+              }).catch(() => {});
+            }
+
+            // mergeable is computed asynchronously and is often null right after
+            // an event, so poll a few times until GitHub has resolved it.
+            let pr = null;
+            for (let i = 0; i < 5; i++) {
+              const { data } = await github.rest.pulls.get({ ...repo, pull_number: number });
+              if (data.mergeable !== null) { pr = data; break; }
+              await new Promise(r => setTimeout(r, 3000));
+            }
+            if (!pr || pr.draft) return;
+            const labels = pr.labels.map(l => l.name);
+
+            if (pr.mergeable === false) {
+              if (labels.includes(READY)) {
+                await github.rest.issues.removeLabel({ ...repo, issue_number: number, name: READY }).catch(() => {});
+              }
+              if (!labels.includes(CONFLICT)) {
+                await github.rest.issues.addLabels({ ...repo, issue_number: number, labels: [CONFLICT] });
+              }
+            } else if (pr.mergeable === true) {
+              if (labels.includes(CONFLICT)) {
+                await github.rest.issues.removeLabel({ ...repo, issue_number: number, name: CONFLICT }).catch(() => {});
+              }
+            }