Update provider documentation with:
- Four provider variants table (zai, zai-coding, zhipu, zhipu-coding)
- Region and plan type guidance
- CLI setup commands for all variants
- Environment variable fallback chains
- Config snippets for each variant
- Important notes on key interchangeability and billing
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via [Happy](https://happy.engineering)
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>
