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.