Pack metadata WP-split: authored pack.yaml/pack.md vs generated pack-manifest.yaml

**Gap #6 · Size S · blocked_by WP-core.**

The pack-layout contract forbids authors editing the generated manifest (`…/contracts/pack-layout.md:104`). Split the concerns into two files rather than fencing an authored block inside a generated one:
- **`pack.yaml`** (authored): `pack_id`, `pack_version`, `parent_pack`, `accompanies_doctrine_pack`, human metadata.
- **`pack.md`** (authored): human-readable pack description.
- **`pack-manifest.yaml`** (generated, never hand-edited): `constituents[]`, content hashes, `manifest_hash`, `charter:` profile.

Per ADR `docs/adr/3.x/2026-08-16-1-pack-metadata-manifest-unification.md` (unify decision) and the disposition `docs/plans/investigations/2026-08-pack-level-metadata-manifest.md`. Parent keystone: #2467. Trust-adjacent: #2539.

Start with the pack-layout contract at …/contracts/pack-layout.md:104, then read ADR docs/adr/3.x/2026-08-16-1-pack-metadata-manifest-unification.md and the disposition docs/plans/investigations/2026-08-pack-level-metadata-manifest.md. Check the dependency on WP-core and parent issue #2467 before changing anything. Done means authored metadata is in pack.yaml and pack.md, while pack-manifest.yaml contains only generated manifest data.

Written by the indexing model from the issue text.