feat: require protected Host capability receipts

[fearclear]

Summary:

• add append-only Host capability policy v2 and protected human-decision receipt workflow
• bind approvals to exact accepted Plugin API ids and contract digests, strict runtime conformance, npm bytes, Host commit, immutable Release, and builder attestation
• update authoring docs and Skill so Plugins must submit Host requests and cannot edit Host implementation or generated Catalogs

Safety:

• receipt consumers reject empty or legacy Catalogs, missing APIs, changed contract digests, forged conformance, bot or self approval, and mutable evidence
• Multi Angle, Panorama Viewer, and Relight Studio remain publication-blocked until real external evidence exists
• no target Plugin source was changed

Verification:

• 119 tooling tests
• repository validation with five expected fail-closed blockers
• generated Skill API reference check
• workflow YAML parsing and diff check

Rollout:

• current vendored Catalog remains wire-schema/2 and SDK provenance is not yet published, so this PR intentionally does not remove the three blockers.

[fearclear]

Replaced by #61, rebuilt from the latest main to avoid conflicts caused by the previously squash-merged branch history. The governance change itself is unchanged and revalidated.