docs: link to OpenRouter docs and remove placeholder content

- Replace local docs link with https://openrouter.ai/docs/features/prompt-caching
- Remove TODO status section with planned filenames (can go stale)
- Remove Expected Usage code block (duplicates actual examples)
- Keep README minimal and maintenance-free

Author: subtleGradient

typescript/openrouter-sdk/src/prompt-caching/README.md

@@ -5,24 +5,8 @@ Examples demonstrating prompt caching with @openrouter/sdk.
 ## Documentation
 
 For full prompt caching documentation including all providers, pricing, and configuration details, see:
-- **[Prompt Caching Guide](../../../../docs/prompt-caching.md)**
+- **[OpenRouter Prompt Caching Guide](https://openrouter.ai/docs/features/prompt-caching)**
 
-## Status
+## Examples in This Directory
 
-**TODO**: Examples coming soon. This directory will contain:
-- `user-message-cache.ts` - Cache large context in user messages
-- `multi-message-cache.ts` - Cache system prompt across multi-turn conversations
-- `no-cache-control.ts` - Control scenario (validates methodology)
-
-## Expected Usage
-
-```typescript
-import OpenRouter from '@openrouter/sdk';
-
-const openrouter = new OpenRouter({
-  apiKey: process.env.OPENROUTER_API_KEY,
-});
-
-// Configuration and cache_control usage pattern will be documented
-// when examples are implemented
-```
+See the TypeScript files in this directory for specific examples.