i have been working on something similar, trying to build the leanest agent loop that can be self modifying. ended up building it as a plugin within OpenCode with the cow pulled out into python hooks that the agent can modify at runtime (with automatic validation of existing behavior). this allows it to create new tools for itself, customize it's system prompt preambles, and of course manage its own traits. also contains a heartbeat hook. it all runs in an incus VM for isolation and provides a webui and attachable TUI thanks to OpenCode.