fix: replace github.sha with gitea.sha in CI workflow

The Gitea Actions workflow used ${{ github.sha }} which is GitHub Actions
syntax. In Gitea Actions the correct context variable is ${{ gitea.sha }}.
This caused the image tag SHA component to be empty.

Closes leeworks-agents/gitea-mobile#20

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -28,7 +28,7 @@ jobs:
         id: tag
         run: |
           TIMESTAMP=$(date +%Y%m%d%H%M%S)
-          SHA=$(echo ${{ github.sha }} | cut -c1-7)
+          SHA=$(echo ${{ gitea.sha }} | cut -c1-7)
           echo "tag=${TIMESTAMP}-${SHA}" >> $GITHUB_OUTPUT
 
       - name: Build Docker image