docs(references): add Anthropic + OpenAI official best practices
- anthropic-prompt-caching.md: KV cache mechanics, TTLs, pricing, auto vs explicit
- openai-prompt-caching.md: automatic caching, in-memory vs 24h retention, prompt_cache_key
- anthropic-prompting-best-practices.md: clear instructions, XML tags, few-shot, model-specific notes
- openai-prompting-best-practices.md: message roles, optimization framework, structured outputs, model selection
Key findings:
- Anthropic caching: only for Claude models, 5m default TTL, 1h optional, 10% cost for reads
- OpenAI caching: automatic/free, 5-10min default, 24h extended for GPT-5+
- GLM/ZAI models: neither caching mechanism applies
- Subagent model routing table added to openai-prompting-best-practices.md
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