Fix invalid Docker tag format in CI/CD

Remove branch prefix from sha-based tags to avoid invalid tag format
when branch name is empty. Tags cannot start with a hyphen.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.github/workflows/deploy.yml

@@ -83,7 +83,7 @@ jobs:
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
-            type=sha,prefix={{branch}}-
+            type=sha
             type=raw,value=staging,enable=${{ github.ref == 'refs/heads/main' }}
             type=raw,value=latest,enable=${{ startsWith(github.ref, 'refs/tags/v') }}