Start an interactive login shell inside the project's agent container image (built by overplane sandbox build). The environment includes the full toolchain, installed agent CLIs, and every host environment variable declared by the configured agent recipes plus agent.container.env_passthrough in overplane.yaml. The working directory is mounted read-only at /project and the overplane binary at /usr/local/bin/overplane. Requires an interactive terminal; the container is removed on exit.