Use generated docs in local circuit prompt

[a1local]

Summary

• Loads https://docs.tscircuit.com/ai.txt as optional generated tscircuit docs for the local circuit system prompt
• Keeps prompt creation resilient when that optional docs endpoint fails
• Adds focused mocked-fetch coverage for the inclusion and fallback paths

Verification

• npx --yes bun test tests/prompt-templates/create-local-circuit-prompt.test.ts
• npx --yes tsc --noEmit
• npx --yes biome check lib/prompt-templates/create-local-circuit-prompt.ts tests/prompt-templates/create-local-circuit-prompt.test.ts
• git diff --cached --check

/claim #45