feat: include auto-generated docs from docs.tscircuit.com/ai.txt in system prompt

[colorbank]

/claim #45

Summary

Fetches auto-generated documentation from https://docs.tscircuit.com/ai.txt and prepends it to the system prompt, giving the AI model up-to-date tscircuit API context.

Changes

• Added fetchOptionalFileContent helper that gracefully returns "" on any error (404 or network failure)
• Modified createLocalCircuitPrompt to fetch ai.txt in parallel with the existing COMPONENT_TYPES.md fetch using Promise.all
• Prepends the generated docs section before ## tscircuit API overview when available; falls back gracefully when unavailable

Tests

Added tests/prompt-templates/create-local-circuit-prompt.test.ts covering:

1. ✅ Generated docs are included when ai.txt is available
2. ✅ Prompt works normally when ai.txt returns 404
3. ✅ Prompt works normally when ai.txt fetch throws
4. ✅ Generated docs appear before the ## tscircuit API overview section