Function run_command_formatter 

pub fn run_command_formatter(
    template: &str,
    abs_path: &str,
    project_root: &str,
) -> Result<(), String>

Run an external formatter command on abs_path with cwd project_root (so tool config like rustfmt.toml is discovered). Returns Err with a clear message if the binary is missing or the command exits non-zero.

Formatter commands are trusted local executables. LeanCTX terminates the formatter and ordinary descendants through a Unix process group or Windows Job Object; a deliberately detached Unix process (setsid) is outside this cooperative cleanup boundary.