Use generated docs in local circuit prompt

[YunHeTracyLee]

Summary

• load the generated tscircuit docs feed from https://docs.tscircuit.com/ai.txt into createLocalCircuitPrompt
• cache the generated docs during a process and fall back gracefully if the docs endpoint fails
• add mocked-fetch coverage for inclusion, fallback, and caching
• make OpenAI-backed tests opt-in when OPENAI_API_KEY is unavailable so default CI remains deterministic

/claim #45

Verification

• npx --yes bun test tests/create-local-circuit-prompt.test.ts
• npx --yes bun test --timeout 50000
• npx --yes bun x tsc --noEmit
• npx --yes bun x biome check lib/prompt-templates/create-local-circuit-prompt.ts tests/create-local-circuit-prompt.test.ts
• npx --yes bun run build
• git diff --check