v0.9.0 — manifest/staleness + multi-doc + _docs bucket

Supersedes the unreleased 0.8.0 draft. Combines manifest + staleness
extraction from that draft with multi-doc support and a top-level
narrative-docs bucket.

Highlights

• Feature.doc_paths: list[str] — a feature may now register multiple docs.
• FeatureManifest.docs: list[str] — top-level _docs: bucket for
  hand-written narrative docs that don't belong to any single feature.
• check_staleness iterates doc_paths, producing one DocStaleness
  entry per registered path.
• Backward compatible: legacy doc_path: (scalar) loads cleanly, migrated
  into a single-entry list.

See CHANGELOG.md for full notes.