sync-fork: use fetch-depth 0 to fix unrelated histories error

Shallow clone (depth=1) lacks the common ancestor between fork and
upstream, causing git pull to fail with "unrelated histories".

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

Author: koki-develop

.github/workflows/sync-fork.yml

@@ -21,6 +21,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
 
       - name: Pull upstream changes
         run: |