Clarify WebContainers licensing and StackBlitz dependency in README

[vivek1504]

Documentation issue

The README should clearly state the dependency on WebContainers (StackBlitz) and the licensing implications.

Requested changes

1. Mention that the project depends on the WebContainers runtime
2. Note that the core runtime is closed-source
3. Link to WebContainers pricing/license page for commercial users
4. Remove or qualify any "zero vendor lock-in" claims