feat: add marketing content skills

[daniellee-ux]

What changed

• add write-marketing-copy for turning source material into truthful, high-retention content packages
• add select-marketing-image for image ranking, evidence boundaries, crop planning, and no-asset fallbacks
• add design-marketing-cover for qualified-attention cover strategy, CoverSpec handoff, and thumbnail audits
• route marketing graphics through the new skills from create-design
• bundle and sync the skills into the demo workspace and generated OpenCanva projects
• update repository and package documentation

Why

OpenCanva could author graphics, but it did not yet package the upstream marketing decisions that determine whether a post earns a click, sustains reading, and remains truthful. These skills make copy, focal-image selection, and cover composition reusable product capabilities.

Impact

Agents can now turn rough source material into a structured marketing workflow:

write-marketing-copy → select-marketing-image → design-marketing-cover → create-design

The skills include Xiaohongshu-specific references while keeping source claims, image rights, privacy, and evidence boundaries explicit.

Validation

• npm run typecheck
• npx tsc --noEmit -p apps/demo
• npm run check:sync