fix(ci): fall back to github.token when GH_TOKEN secret is not set

When secrets.GH_TOKEN is not configured, ${{ secrets.GH_TOKEN }} evaluates
to an empty string and actions/checkout@v4 fails with:
  "Input required and not supplied: token"

Using ${{ secrets.GH_TOKEN || github.token }} ensures the workflow always
has a valid token. GH_TOKEN (PAT) is preferred when available since it can
push branches and create PRs; github.token is used as a safe fallback.

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

.github/workflows/regenerate-poetry-lock.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          token: ${{ secrets.GH_TOKEN }}
+          token: ${{ secrets.GH_TOKEN || github.token }}
 
       - name: Set up Python
         uses: actions/setup-python@v5
@@ -74,4 +74,4 @@ jobs:
             --head "$BRANCH" \
             --base main
         env:
-          GH_TOKEN: ${{ secrets.GH_TOKEN }}
+          GH_TOKEN: ${{ secrets.GH_TOKEN || github.token }}