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.

Create the root-level VISION.md and review the existing documentation navigation to find where it should be linked. The work is done when the document presents a changeable working vision without roadmap, schedule, guarantee, obligation, or commitment language, and is discoverable from the documentation site.

Written by the indexing model from the issue text.