Add CHANGELOG.md modification check and warning comment

.github/actions/post-comment/action.yml

@@ -0,0 +1,41 @@
+name: "Post comment"
+description: "Create or update a sticky PR comment identified by a caller-provided marker."
+inputs:
+  marker:
+    description: >
+      Non-empty plain-text identifier for the comment. Use a unique, stable value
+      for each distinct comment; it is automatically wrapped in a hidden HTML comment.
+    required: true
+  body:
+    description: "Full comment markdown."
+    required: true
+  update-only:
+    description: Do nothing if no comment with the specified marker exists.
+    required: false
+    default: "false"
+  issue-id:
+    description: >
+      The issue or pull request number to post/update the comment on.
+      Defaults to the current pull request if not provided.
+    required: false
+  token:
+    description: "The GitHub token to authenticate with."
+    required: true
+runs:
+  using: composite
+  steps:
+    - name: Post comment
+      shell: bash
+      env:
+        BODY: ${{ inputs.body }}
+        MARKER: ${{ inputs.marker }}
+        UPDATE_ONLY: ${{ case(inputs.update-only == 'true', '--update-only', '') }}
+        GH_TOKEN: ${{ inputs.token }}
+        ISSUE_ID: ${{ inputs.issue-id || github.event.pull_request.number }}
+      run: |
+        npx tsx ./pr-checks/post-comment.ts \
+          --body "$BODY" \
+          --marker "$MARKER" \
+          $UPDATE_ONLY \
+          --issue-id "$ISSUE_ID" \
+          --repository "$GITHUB_REPOSITORY"

pr-checks/api-client.ts

@@ -8,3 +8,14 @@ export function getApiClient(token: string): ApiClient {
   const opts = githubUtils.getOctokitOptions(token);
   return new githubUtils.GitHub(opts);
 }
+
+/** Resolves the GitHub API token to use, from `GH_TOKEN` or `GITHUB_TOKEN`. */
+export function resolveToken(env: NodeJS.ProcessEnv): string {
+  const token = env.GH_TOKEN?.trim() || env.GITHUB_TOKEN?.trim();
+  if (!token) {
+    throw new Error(
+      "Missing authentication token. Set GH_TOKEN or GITHUB_TOKEN.",
+    );
+  }
+  return token;
+}