ci: only tag Docker image as "latest" on main branch

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

Author: tonyxiao

.github/workflows/ci.yml

@@ -890,4 +890,4 @@ jobs:
         run: bash scripts/promote-to-dockerhub.sh
         env:
           GHCR_IMAGE: ghcr.io/${{ github.repository }}:${{ github.sha }}
-          DOCKERHUB_TAGS: ${{ steps.tag.outputs.name }} latest
+          DOCKERHUB_TAGS: ${{ steps.tag.outputs.name }}${{ github.ref_name == 'main' && ' latest' || '' }}