improve: highlight active-development stability

[samohovets]

Summary

• Add a prominent active-development warning near the README introduction.
• Clarify that schemas, concepts, source-editing behavior, wire formats, and public APIs can change without compatibility guarantees.

Changelog

Public summary:

• The README now clearly highlights Patchwave's experimental, macOS-first status and current lack of compatibility guarantees.

Context:

• Helps prospective adopters see the risk of breaking changes before following the Quick Start.
• Closes Add an active-development stability notice to the README #5.

Verification

• git diff --check
• pnpm exec oxfmt --check README.md