make hnt-agent architecture more headless

[veilm]

actually hnt-edit too
neither of them should ever interact with the EDITOR or prompt you directly, maybe

then you have a wrapper that does the UI loop

actually this made more sense before rust. now they're more interlinked internally, but you at least generally want this to be possible through the exposed CLI
because you want it to be possible for hnt-agent to use both hnt-agent and hnt-edit headlessly if needed

probably make sure that each --help works quite well, or have some specific hnt-help that just writes some documentation. or wrap help inside headlesh so that help hnt-edit gives some specific LLM-friendly 80/20 docs?
basically you want to avoid polluting the original system prompt, and only mention what each tool does conceptually, rather than how