Function build_inputs_from_dirs 

pub fn build_inputs_from_dirs(
    project_root: &Path,
    explicit_dir: Option<&Path>,
    from_hint: RuleFormat,
) -> Result<Vec<InputRule>, Error>

Build InputRule entries by scanning tool directories or an explicit path.

When explicit_dir is None, scans crate::discover::existing_tool_dirs under project_root (skipping canonical .agents/rules as a source).