Remove cloud embedding config and add just tasks

[llbbl]

Summary

• remove Cloudflare/Gemini/OpenAI references and env/provider plumbing to keep embeddings local only
• simplify rate limit proxy header support and update search/index to hardcode local embeddings
• replace the Makefile with a justfile and document the new recipes
• run test coverage in CI and upload reports to Codecov

Testing

• pnpm format
• pnpm lint
• pnpm test
• pnpm exec tsc --noEmit