ci: gate element-hq-only workflows on fork

Skip upstream deploy/publish/release, org automation, translations and
PR previews via a repo guard; allow PRs to release/* base branches.

.github/workflows/release_prepare.yml

@@ -15,6 +15,7 @@ on:
 permissions: {} # Uses ELEMENT_BOT_TOKEN instead
 jobs:
     checks:
+        if: ${{ github.repository == 'element-hq/element-web' }}
         name: Sanity checks
         strategy:
             matrix:
@@ -28,6 +29,7 @@ jobs:
             repository: ${{ matrix.repo }}
 
     prepare:
+        if: ${{ github.repository == 'element-hq/element-web' }}
         runs-on: ubuntu-24.04
         needs: checks
         env: