Function stage_apply 

pub fn stage_apply(
    submission: BuilderSubmission,
    config: &mut Config,
    surface: Surface,
) -> Result<StagedQuickstartApply, Vec<QuickstartError>>

Validate one submission and stage it onto a working config clone: every mutation (provider, presets, channels, peer groups, agent, completion flag) plus personality-file staging to tempfiles. No irreversible work happens here — a rejected submission returns with nothing on disk changed, and dropping the returned staging (or never completing it) cleans up its temp files.