chore(opencode-sandbox): add SSH auth and workspace mount for Gitea

- Mount host workspace at /home/ga/workspace
- Add SSH key and known_hosts for Gitea auth
- Configure git user identity in container

.opencode-sandbox/docker-compose.yml

@@ -10,7 +10,10 @@ services:
       - NET_RAW
     volumes:
       # Your monorepo — opencode's working directory
-      - ../:/workspace
+      - /home/ga/workspace:/workspace
+      # SSH key for Gitea auth
+      - ~/.ssh/id_ed25519:/root/.ssh/id_ed25519:ro
+      - ~/.ssh/known_hosts:/root/.ssh/known_hosts:ro
       # Persist opencode auth so you don't re-login every time
       - opencode-auth:/root/.local/share/opencode
     environment: