Function observer_for_config 

pub fn observer_for_config(config: &Config) -> Option<Arc<FormatObserver>>

Build the FormatObserver a fresh crate::Agent should install, given a resolved crate::Config — called once, from crate::agent::build_tool_context. Config::formatters_enabled is the ONE gate: false (the default) returns None — no formatter ever runs, byte-identical to before this module existed.