Public models are now display-only and don't require a Model ID:
- Removed Model ID input field from admin.html public models section
- Updated admin.js to not require or send Model ID for public models
- Updated server API to accept public models without Model ID
- Uses display label as the identifier for public models
- Only requires Model ID for OpenCode models (which need it for execution)
Public models are purely for user-facing display in the builder dropdown,
while OpenCode models form the actual execution fallback chain.
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