chore(opencode-sandbox): fix SSH key permissions and remove read-only flag

.opencode-sandbox/docker-compose.yml

@@ -12,7 +12,7 @@ services:
       # Your monorepo — opencode's working directory
       - /home/ga/workspace:/workspace
       # SSH key for Gitea auth
-      - ~/.ssh/id_ed25519:/root/.ssh/id_ed25519:ro
+      - ~/.ssh/id_ed25519:/root/.ssh/id_ed25519
       - ~/.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