[dajiaohuang]

Summary

• Use the exact proposal_packet narrative-structure enum spellings and list all supported values in the explainer idea guidance.
• Show brief.target_platform as one allowed value instead of a pipe-delimited string.
• List the complete delivery_promise.promise_type enum in the cinematic proposal example.

This is a focused documentation follow-up to #493's schema-vocabulary mismatch report. It does not close the remaining items in that issue.

Validation

• git diff --check passed.
• Tests not run; this change only updates guidance examples and enum vocabulary.