Use built-in GITHUB_TOKEN for Gitea registry login

Avoids needing to configure REGISTRY_USER / REGISTRY_PASSWORD secrets.
Gitea injects a per-run token scoped to the repo's own packages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/flow.yml

@@ -42,9 +42,9 @@ jobs:
 
       - name: Log in to Gitea registry
         run: |
-          echo "${{ secrets.REGISTRY_PASSWORD }}" | \
+          echo "${{ secrets.GITHUB_TOKEN }}" | \
             docker login ${{ env.REGISTRY }} \
-              --username ${{ secrets.REGISTRY_USER }} \
+              --username "${{ github.actor }}" \
               --password-stdin
 
       - name: Build image