Add a working vision document

## Summary

Add a root-level `VISION.md` that describes the project's working vision for Spec Kit and agentic software development.

## Notes

The document should be clear that it is a working vision maintained by the Spec Kit maintainers. It should not be framed as a roadmap, release schedule, guarantee, obligation, or commitment. It should also state that the content can change, be replaced, or be removed at any time.

## Suggested docs integration

Reflect the root-level vision document in the docs navigation so it is discoverable from the documentation site.

在根目录创建 VISION.md，并检查现有的文档导航，以确定应在哪里添加链接。当该文档呈现一个可变更的工作愿景，不包含路线图、时间表、保证、义务或承诺相关的措辞，并且可以从文档网站中找到时，这项工作就完成了。

由索引模型根据 Issue 内容生成。