Function archive_phase_files 

pub fn archive_phase_files(
    project_root: &Path,
    evidence_root: &Path,
    phase: u32,
    retain: usize,
) -> Result<Option<String>, Error>

Archive the prior stage’s stdout/exit captures into bounded per-phase history instead of wiping them outright, so a false-positive self-report can be diagnosed after the fact (16b). Replaces the old cleanup_phase_files, which deleted these files unconditionally.

At most retain capture generations are kept per phase; older ones are pruned (see [prune_history]). The agent-pid file is still removed outright — it is process bookkeeping, not diagnostic output. When there is nothing to archive (first launch), this is a no-op success.