Initial scaffold: Debian-based opencode v1.4.0 dev container

Dockerfile with Node.js 22, git, ssh, fzf, ripgrep, fd, non-root user.
Entrypoint auto-configures provider from env vars.
docker-compose with workspace mount, SSH keys, and persistent data volume.

.dockerignore

@@ -0,0 +1,5 @@
+.git
+.env
+*.md
+LICENSE
+.DS_Store