fix(ci): skip backport update job on fork PRs

Fork PRs don't have access to repository secrets, so the GitHub App
token generation fails and shows a red X on every fork PR. Skip the
update-backport job entirely when the PR comes from a fork, since
it cannot function without the app token anyway.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Saadnajmi

.github/workflows/microsoft-backport.yml

@@ -194,7 +194,10 @@ jobs:
   # ─── Job 2: Auto-update backport PRs when source PR is updated ───
   update-backport:
     name: Update backport PRs
-    if: github.event_name == 'pull_request' && github.event.action == 'synchronize'
+    if: >
+      github.event_name == 'pull_request' &&
+      github.event.action == 'synchronize' &&
+      github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name
     runs-on: ubuntu-latest
     permissions:
       contents: write