Add prompt-caching examples for Effect AI (#44)

Add prompt-caching examples for Effect AI

- Add typescript/effect-ai/src/prompt-caching/user-message-cache.ts
- Demonstrates cache_control using options.openrouter.cacheControl in Prompt
- Shows Effect.gen pattern with Layer-based dependency injection
- Critical configuration: stream_options.include_usage in model config layer
- Evidence-based verification via response.usage.cachedInputTokens

Run biome format

Simplify Effect AI prompt-caching README to link to main docs

Rename prompt caching examples with anthropic prefix