Add publish workflow and daytona sandbox images
Add GitHub Actions workflow to build and push daytona/* Docker images
to ghcr.io using a matrix of sandboxes and Buildx caching. Images are
tagged 0.6.0. Add Dockerfiles for multiple sandboxes (amp, claude,
codex, copilot, crush, gemini, mise, kilo, kiro, nix, nullclaw, pkgx,
openclaw, opencode, opencrust, picoclaw, zeroclaw)