Filter new-agent model list by selected provider

[kocendavid]

Summary

• load available models for the spawn wizard from '/api/models?available=true'
• filter model options by selected provider in the New Agent modal
• auto-sync selected model when provider changes
• keep manual model input fallback when provider has no available models

Why

The Create Agent flow should only show models that are actually available for the selected provider to avoid invalid combinations and reduce user error.

Testing

• Open Create Agent modal (wizard mode)
• In Step 2, change provider and verify model list updates to provider-specific available models
• Verify providers with no available models show manual model input fallback
• Spawn agent successfully with selected model