schema: publish via GitHub Pages and migrate URI to pages path

[a-a-k]

Summary

• add GitHub Pages schema publishing workflow (schema-v* tags)
• migrate schema URI to https://mb3r-lab.github.io/Bering/schema/model/v1.0.0/model.schema.json
• update pinned digest after schema $id change
• add guard tests for $id/version-path binding
• update docs and regenerated example outputs

Validation

• go test ./...

Notes

• This intentionally introduces temporary contract drift with Sheaft until Sheaft updates pinned URI/digest in its own repo.