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/entrypoint.sh

@@ -1,2 +1,5 @@
 #!/bin/bash
+export GIT_SSH_COMMAND="ssh -i /root/.ssh/id_ed25519 -o StrictHostKeyChecking=accept-new"
+git config --global user.email "gabriel.pereira@protonmail.com"
+git config --global user.name "gabspereira"
 exec opencode