Add fork PR blocking for private runner in workflow

.github/workflows/Run_RPS_AppKit-E8.yml

@@ -20,15 +20,51 @@ on:
   workflow_run:
     workflows: ["Build RPS for AppKit-E8 Hardware"]
     types: [completed]
-    branches: [main]
+    branches-ignore: ["main"]
 
 permissions:
   contents: read
   actions: read
 
 jobs:
+  fork-pr-blocked:
+    name: Block fork PRs from private runner
+    if: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.head_repository.full_name != github.repository }}
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+
+    steps:
+      - name: Notify runner restriction for fork PRs
+        run: |
+          echo "Workflows on the private runner are restricted for security reasons."
+          echo "Contributor can validate PRs using GitHub-hosted runners."
+          exit 1
+
+      - name: Comment on PR
+        if: ${{ github.event.workflow_run.pull_requests[0].number }}
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const pr = context.payload.workflow_run.pull_requests[0].number;
+
+            await github.rest.issues.createComment({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: pr,
+              body: [
+                "⚠️ **AppKit-E8 hardware test was not run**",
+                "",
+                "This PR comes from a fork, so it is intentionally blocked from using the private runner.",
+                "",
+                "A maintainer must run the hardware workflow from the base repository if hardware validation is required."
+              ].join("\n")
+            });
   CI:
-    if: ${{ github.event_name == 'workflow_dispatch' || (github.event.workflow_run.conclusion == 'success') }}
+    if: >-
+      ${{ github.event_name == 'workflow_dispatch' ||
+      (github.event.workflow_run.conclusion == 'success' &&
+      github.event.workflow_run.head_repository.full_name == github.repository) }}
     runs-on: [self-hosted, rsp-p5-01]
 
     steps: