Include generated docs in local circuit prompt

[jorgeschulz]

Summary

/claim #45

• includes the generated tscircuit docs from https://docs.tscircuit.com/ai.txt in createLocalCircuitPrompt
• loads the required props docs and optional generated docs in parallel
• caches generated docs during the process so benchmark prompt creation does not refetch the same generated docs repeatedly
• keeps prompt generation resilient when the generated docs endpoint is unavailable
• adds mocked-fetch coverage for generated-doc inclusion and fallback behavior
• makes OpenAI-backed tests opt-in when OPENAI_API_KEY is present, so the default test run is deterministic without credentials

Verification

• npx --yes bun@latest test tests/prompt-templates/create-local-circuit-prompt.test.ts --timeout 50000
• npx --yes bun@latest test --timeout 50000
• npx --yes bun@latest x tsc --noEmit
• npx --yes bun@latest x biome check lib/prompt-templates/create-local-circuit-prompt.ts tests/prompt-templates/create-local-circuit-prompt.test.ts tests/tscircuit-coder.test.ts tests/utils/generate-random-prompts.test.ts
• npx --yes bun@latest run build
• git diff --check

AI-assisted with Codex; I reviewed the diff and ran the checks above before opening this PR.