website_repo Astro loaders → plan.json native (§35.7 / Decision 14)

Rewire website_repo's Astro plan-rendering pipeline to consume plan.json natively (per Decision 14: plan.json is the SOLE canonical plan artifact) instead of reading per-section section-*.md frontmatter. Downstream of scripts-first-workflow-architecture §35.7 — the live v7 content node plans/completed/scripts-first-workflow-architecture/content/json-only-architecture-and-rendered-sidecar-cleanup--s-32a7ca8a.md (pre-v7 source archived at _archive/section-35-json-only-architecture-and-rendered-sidecar-cleanup.md). The §35.9 corpus self-migration (now tracked inside that same v7 plan, with the website-side full-cutover step owned by its content/website-repo-migration--s-6ccb6456.md §10 node) produces the plan.json files this consumer reads.