docs(provider): 补充多协议注册表与模型路由设计

[yovinchen]

Linked issue

Closes #723

Summary

• 记录 ProviderDefinition、ProviderInstance、ProviderInterface 与 ProviderModel 的目标分层。
• 设计 OpenAI Chat、Responses、Anthropic Messages、Gemini 等多协议接口与模型级绑定。
• 明确模型能力、输入输出模态、限制、价格、媒体任务和受信任客户端档案的边界。
• 补充兼容迁移、凭证存储、故障转移与分阶段实施建议。
• 在文档索引中增加 Provider Registry 设计入口。

Change scope

• Modules: documentation
• Key paths: docs/README.md, docs/design/provider-registry-and-model-routing.md

Screenshots / preview

N/A — 本 PR 仅新增架构设计文档与索引，不改变运行时界面。

Verification

• git diff --check origin/main...HEAD — passed
• 文档内 4 个源码相对链接均已核验存在
• 文档索引目标文件已核验存在

Pre-submit checklist

• Scope is documentation-only and linked to the corresponding issue.
• Proposal status is clearly marked as not implemented.
• Relative source links and index entry are valid.
• No Workbench implementation files are included.

[github-actions]

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