docs: 重写 README 介绍产品与开发指南

[N123999]

Summary

• Rewrite the root README with Hero, monorepo architecture, and Agent Core capabilities (runtime, transports, host tools, MCP/Skills/Rules).
• Replace the Desktop README WIP placeholder with product positioning, feature overview, requirements, and development instructions.

Test plan

• Open root README.md and apps/desktop/README.md on GitHub and verify Markdown rendering.
• Confirm internal anchor links (Desktop, CLI, Agent Core, Development) resolve correctly.

[N123999]

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^{Reviewed by Cursor Bugbot for commit f6c82c2. Configure here.}