Strengthen Office artifact pillar

[Dkm0315]

Summary

• Add artifact_structural_verify and office_artifact_contract to the artifact-studio capability pack.
• Add muster artifacts contract and muster artifacts verify plus structural verification output after artifact creation.
• Make muster integrations sample artifact-studio create and verify a real DOCX sample.
• Surface Office artifacts as a first-class README pillar while staying explicit that polished visual QA needs app-server skills or local renderers.

Validation

• pnpm exec tsx --test packages/core/test/integration-packs.test.ts
• pnpm exec tsx --test packages/cli/test/cli.test.ts
• pnpm build
• pnpm exec tsx --test packages/core/test/qa-pack-readiness.test.ts packages/core/test/capability.test.ts
• Manual smoke: muster artifacts contract, muster artifacts create, muster artifacts verify, muster integrations workflow artifact-studio, muster integrations sample artifact-studio

Notes

• This does not claim polished Office layout/render QA from the lightweight local builders. Those paths are still routed through document/spreadsheet/presentation/PDF app-server skills or local renderers when available.