updates: improve NCP validation workflow and adopter docs

[jhull]

Summary

• align schema/docs/examples for reliable NCP interchange
• add scope-aware storybeat sequence validation in schema (signpost/progression/event)
• split canonical fixtures from legacy migration exports
• add reusable file validator, validation guide, and CI workflow for adopters
• tighten onboarding docs for self-serve validation

Validation

• npm run validate:schema
• npm run validate:file -- examples/example-story.json examples/anora.json examples/the-shawshank-redemption.json