Function config_fields_block 

pub fn config_fields_block() -> String

Generates the commented fields = [...] block for the default config file.

Used by both config-generation paths — default_config_content() in main.rs (full write) and Config::merge_defaults in config.rs (merge missing) — so the two can no longer drift apart. Emits every field key from [FIELDS], wrapped to a readable width, all commented out.