ci(upstream-sync): skip LFS smudge to unblock fetch step

The repo's .gitattributes (inherited from upstream) routes certain paths
through git-lfs. Gitea's LFS store doesn't hold those blobs, so on
checkout the smudge filter tries to download them, 404s, and leaves git
in a state where subsequent 'git fetch' calls appear to succeed but
don't actually populate refs.

Run 89 was bitten by this: checkout 'succeeded' with an LFS smudge
fatal, then 'git fetch upstream develop' ran silently, 'git merge
--ff-only upstream/develop' failed because upstream/develop ref
didn't exist locally, and the workflow logged a misleading warning
blaming a divergence that wasn't there.

Setting GIT_LFS_SKIP_SMUDGE=1 keeps LFS pointers as-is. We don't need
image bytes to ff-merge and diff refs.

.gitea/workflows/upstream-sync.yml

@@ -17,6 +17,13 @@ on:
 jobs:
   sync-main:
     runs-on: ubuntu-latest
+    env:
+      # The repo's .gitattributes (inherited from upstream) routes the
+      # screenshots/ tree through git-lfs. Gitea's LFS store doesn't hold
+      # those blobs, so on checkout the smudge filter tries to 404-download
+      # them and wedges git state for subsequent fetches. We don't need
+      # the image bytes here — leave LFS pointers as-is.
+      GIT_LFS_SKIP_SMUDGE: '1'
 
     steps:
       - name: Checkout main
@@ -24,6 +31,7 @@ jobs:
         with:
           ref: main
           fetch-depth: 0
+          lfs: false
           # Built-in token Gitea hands us — scoped to this repo, has push.
           token: ${{ secrets.GITEA_TOKEN }}