Function validate_agents 

pub fn validate_agents(
    config: &Config,
    root: &Path,
) -> (Vec<String>, Vec<String>)

Layer 1 of the two-layer manifest validation design. Validates all configured agent names and scans .apm/agents/ for issues. Returns (errors, warnings) so callers can route them — keeps this single directory scan from running twice when both validate_config and validate_warnings are invoked.