Function write_supervisor_summary 

pub fn write_supervisor_summary<S: BuildHasher>(
    state: &BrokerState,
    session: &Session,
    merge_order: &[String],
    test_results: &HashMap<String, TestResult, S>,
    repo_root: &Path,
) -> Result<PathBuf, PawError>

Convenience wrapper that writes the session summary to a timestamped file under <repo_root>/.git-paw/sessions/<UTC-timestamp>.md, creating .git-paw/sessions/ if it does not already exist.

Each supervisor run produces a fresh file so multiple sessions for the same repo can coexist on disk without overwriting each other. The timestamp format is YYYY-MM-DDTHH-MM-SSZ (filesystem-safe — colons in ISO 8601 are replaced with hyphens) and is taken from the system clock at write time.

Returns the path of the file that was written so callers can log it.