fix(ci): use REGISTRY_TOKEN secret for container registry auth

The automatic GITHUB_TOKEN lacks package write permissions. Switch to
a manually configured PAT stored as REGISTRY_TOKEN.

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

.gitea/workflows/build-package.yaml

@@ -24,7 +24,7 @@ jobs:
         with:
           registry: git.sneakygeek.net
           username: ${{ gitea.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Extract metadata (tags, labels)
         id: meta