feat: require pull requests to follow template

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

@@ -0,0 +1,80 @@
+name: Check PR Template
+
+on:
+  pull_request_target: # zizmor: ignore[dangerous-triggers]
+    types: [opened, edited]
+
+permissions: {}
+
+jobs:
+  parse:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.pull_request.head.repo.fork == true }}
+    permissions:
+      contents: read
+    outputs:
+      uses_template: ${{ steps.check.outputs.uses_template }}
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          sparse-checkout: .github/pull_request_template.md
+          sparse-checkout-cone-mode: false
+          persist-credentials: false
+
+      - name: Check required sections
+        id: check
+        env:
+          BODY: ${{ github.event.pull_request.body }}
+        run: |
+          OK=true
+          while IFS= read -r header; do
+            printf '%s\n' "$BODY" | grep -qF "$header" || OK=false
+          done < <(grep "^## " .github/pull_request_template.md)
+          echo "uses_template=$OK" >> "$GITHUB_OUTPUT"
+
+  act:
+    runs-on: ubuntu-latest
+    needs: parse
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Close PR
+        if: ${{ needs.parse.outputs.uses_template == 'false' && github.event.pull_request.state != 'closed' }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+          NODE_ID: ${{ github.event.pull_request.node_id }}
+        run: |
+          gh api graphql \
+            -f prId="$NODE_ID" \
+            -f body="This PR has been automatically closed as the description doesn't follow our template. After you edit it to match the template, the PR will automatically be reopened." \
+            -f query='
+              mutation CommentAndClosePR($prId: ID!, $body: String!) {
+                addComment(input: {
+                  subjectId: $prId,
+                  body: $body
+                }) {
+                  __typename
+                }
+                closePullRequest(input: {
+                  pullRequestId: $prId
+                }) {
+                  __typename
+                }
+              }'
+
+      - name: Reopen PR (sections now present, PR closed)
+        if: ${{ needs.parse.outputs.uses_template == 'true' && github.event.pull_request.state == 'closed' }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+          NODE_ID: ${{ github.event.pull_request.node_id }}
+        run: |
+          gh api graphql \
+            -f prId="$NODE_ID" \
+            -f query='
+              mutation ReopenPR($prId: ID!) {
+                reopenPullRequest(input: {
+                  pullRequestId: $prId
+                }) {
+                  __typename
+                }
+              }'