docs: align project structure documentation with actual codebase

[Copilot]

The README documented non-existent packages (mcp/, webui/) while omitting actual ones (shared-model/, plugins/provider-openai/).

Changes

Root README.md

• Updated project structure tree to reflect actual monorepo layout
• Corrected package table: replaced mcp and webui with shared-model and provider-openai
• Updated documentation navigation to reference existing files only
• Removed "MCP extension support" from core features list

Package documentation

• Added packages/shared-model/README.md describing unified model abstraction and provider interfaces
• Added plugins/provider-openai/README.md documenting OpenAI provider plugin functionality and configuration

All markdown files formatted with prettier per project standards.

Original prompt

给这个项目按照原来介绍的格式更新新的介绍

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[Copilot]

Pull request overview

This PR aligns the project documentation with the actual monorepo structure by removing references to non-existent packages and adding documentation for existing ones.

Key changes:

• Removed documentation references to non-existent mcp and webui packages
• Added comprehensive README files for shared-model and provider-openai packages
• Updated project structure diagram and package table to reflect actual codebase layout

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Updated project structure tree, package table, and documentation navigation; removed references to non-existent packages (mcp, webui) and documentation files (DESIGN.md, extensions/README.md); removed "MCP extension support" from features list
packages/shared-model/README.md	New documentation describing the shared model abstraction package with correct cross-references to core and provider-openai packages
plugins/provider-openai/README.md	New documentation for the OpenAI provider plugin with accurate dependency links and package information

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After thoroughly reviewing this pull request, I found no issues with the changes. The documentation has been accurately aligned with the actual codebase:

✅ All package names match their respective package.json files
✅ All file path references point to existing files
✅ All relative links are correctly constructed
✅ The project structure tree accurately reflects the directory layout
✅ Cross-references between packages are bidirectional and correct
✅ Removed features and documentation references that don't exist in the codebase
✅ Markdown formatting is consistent across all files

The PR successfully achieves its stated goal of documenting the actual project structure while removing references to non-existent components.

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