Add Dockerfile and .dockerignore for Lemonslice agent; update agent prompts and configuration files (#66)

- Introduced a Dockerfile to build a minimal container for the Lemonslice agent, utilizing UV for dependency management and a non-privileged user for security.
- Added a .dockerignore file to exclude unnecessary files from the Docker context.
- Updated agent prompts to refine role instructions and critical switching rules for better user interaction.
- Created a new livekit.toml configuration file for agent settings.