Function render_config 

pub fn render_config(choice: &LadderChoice) -> String

Render a complete, runnable firstpass.toml for these answers — including the [[provider]] and [[gate]] blocks the choice requires. Emitting a fragment would be worse than emitting nothing: only anthropic/openai are built-in providers and only non-empty/json-valid are built-in gates, so anything else must be declared here or the file will not resolve.