docs(creative): clarify preview_creative renders manifests, not generates

[bokelley]

Closes #4452

The preview_creative task reference page opened with "Generate preview renderings" and its frontmatter description said "generates visual previews" — both misleading, since the task renders an existing manifest into viewable output and never generates or modifies it. Implementers choosing between preview_creative and build_creative had no explicit statement of the render/generate distinction on either page.

Changes:

• preview_creative.mdx: frontmatter description changes "generates" → "renders an existing creative manifest into viewable output"; opening sentence replaced with an explicit render/generate distinction and cross-link to build_creative
• build_creative.mdx: cross-link to preview_creative folded into the existing opening sentence so readers see the downstream render step immediately

Non-breaking justification: docs-only wording fix — no schema fields, enum values, or task definitions touched; existing consumers unaffected.

Pre-PR review:

• code-reviewer: approved — no blockers; nit on build_creative.mdx opening sentence redundancy addressed in final diff
• docs-expert: approved — accurate, agent-parseable, cross-links resolve to real files; nit on preview_creative.mdx "input manifest" wording incorporated

Triage-managed PR. This bot does not currently iterate on
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issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
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  issue.

See #3121
for context.

Session: https://claude.ai/code/session_01Ak6rrT9p3ZzeU6YY2y9o5p

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[bokelley]

Applying claude-triaged label to this PR per triage routine convention.

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