docs(flows): extend the PRD to five guided creation flows

One SetupFlow shell with four shared pieces and a persisted setup stage
per document. Storyboard stays specified in full. Video, Script, Image
and Workflow are specified to phase level: steps, plan shape, presets,
generate action, landing, data fields, tools, acceptance criteria.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NbKxMtVCMjprh9UMMaxjYu

Author: claude