Function write 

pub fn write(
    cache_root: &Path,
    repo_path: &Path,
    key: &str,
    entry: &CachedNarrative,
)

Write entry to the sidecar for key, creating the enrichment cache directory if needed.

Best-effort: a cache is an optimization, so a directory-creation, serialization, or write failure is logged at warn and swallowed — a run must never fail because its advisory narrative could not be cached.