feat(client): typed render builders for preview_creative discriminator

[bokelley]

Summary

Adds urlRender / htmlRender / bothRender factory helpers + a grouped Render namespace — directly targets the preview_creative drift class matrix v13 surfaced (2 hits on /previews/0/renders/0/preview_url: must have required property 'preview_url').

Mirrors the asset-builder pattern shipped in #771. PreviewRender is a oneOf on output_format with three variants (url / html / both); the discriminator decides which sibling becomes required. Matrix runs kept catching renders that dropped either the discriminator or the sibling it makes required.

// Before — easy to forget output_format or preview_url
renders: [{ render_id: 'r1', role: 'primary', preview_url: 'https://preview/r1' }]
// ↑ fails validation: missing `output_format`

// After — helper injects the discriminator; input type requires preview_url
renders: [urlRender({ render_id: 'r1', role: 'primary', preview_url: 'https://preview/r1' })]

Design

• Three helpers, one per variant. Can't call urlRender({...}) without preview_url because its input type requires it.
• Return type Omit<Variant, 'output_format'> & { output_format: <literal> } — regen-stable whether or not the generated interface declares the discriminator in the future.
• Discriminator spread last so a runtime cast that slips output_format: 'wrong' into the input still produces the correct tag.
• Render namespace (Render.url(...)) as convenience for renders[] arrays that mix variants.

Skill updates

build-creative-agent and build-generative-seller-agent cross-cutting pitfall callouts now recommend the render helpers alongside the asset helpers, completing the set:

• imageAsset / videoAsset / htmlAsset / urlAsset for each asset
• urlRender / htmlRender / bothRender for each render

Both classes of drift the matrix has been catching now have typed construction paths Claude can lean on.

Test plan

• npm run typecheck clean
• 5 subtests under render builders — discriminator injection per variant, field forwarding, spread-order invariant, namespace equivalence
• npm test — no regressions
• Matrix v14 (post-merge) measures whether the preview_creative drift class clears

Minor version bump — new public API, no breaking change.