fix(storyboards): use literal creative_id in creative_fate reassign (closes #2850)

[bokelley]

Summary

Fixes #2850. The reassign_creative step in creative_fate_after_cancellation.yaml referenced $context.creative_id, a key populated from the seller's sync_creatives response at creatives[0].creative_id. Sellers whose envelope doesn't surface that exact path resolved to undefined, the template engine stripped the creative entry, and the final request failed pre-flight zod with "creatives: expected array, received undefined" — before reaching the agent.

Uses the buyer-set literal acme_reuse_banner_001 (from phase 1's sync_creative_with_assignment) instead. Buyer-authoritative; robust against envelope variance.

Test plan

• Compliance storyboard for this scenario now reaches the reassign_creative step without SDK pre-flight rejection (verified locally against the adcp-1 training agent).