docs(readme): add Agent-native architecture section

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Summary

Add a bilingual Agent-Native Architecture section to the READMEs, placed directly after the hero screenshots.

Changes

• New fragments docs/fragments/_agent-native.md and _agent-native.zh-CN.md.
• Wired into root, vue, react, and angular templates (en + zh-CN where applicable).
• Regenerated all READMEs (pnpm docs:generate); pnpm docs:check passes.

Content

The section states the project's Agent positioning in six points:

1. Agent and user are equal citizens with identical entry points.
2. The Agent serves the chart and user interaction, not a stream of text.
3. AI-Native architecture: tools are woven into chart API primitives as aspects, not a parasitic layer (Zed native vs VS Code extension-based contrast).
4. The Agent work pushes the architecture toward SSOT consistency and testability.
5. MCP is deliberately not used; the project moved from JSON config to MCP and finally to an AI-Native design.
6. StateKernel is the foundation that lets the Agent share the user's exact state at near-zero cost, with tools as a subset of actions.