1. Landing Page (docs/index.md)

• [x] 1.1 Rewrite hero text and tagline to describe OpenSpec rendering (not OpenAPI)
• [x] 1.2 Replace "API Reference" action link with a link to /openspec/
• [x] 1.3 Rewrite feature cards: Spec Pages, Change Pages, Navigation Helpers
• [x] 1.4 Remove all references to OpenAPI, Swagger, endpoints, and tag-grouped sidebar

2. Verify

• [x] 2.1 Run bun run docs:build — build must succeed without errors