Add prompt-caching placeholder for OpenRouter SDK

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Add prompt-caching placeholder for OpenRouter SDK

• Add typescript/openrouter-sdk/src/prompt-caching/README.md
• Placeholder for future @openrouter/sdk examples
• Will follow same pattern as other SDKs (separate files per scenario)

Simplify OpenRouter SDK prompt-caching README to link to main docs

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• #46
• #45 👈 (View in Graphite)
• #52 : 3 other dependent PRs (#47 , #48 , #49 )
• #44
• #43 : 1 other dependent PR (#51 )
• #50
• #42
• #41
• main

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