fix: claw setup writes model at wrong JSON level

[TheArchitectit]

Summary

• Fix claw setup wizard writing model inside the provider object instead of at the top level of settings.json
• parse_optional_model reads from the top level, so the model setting was silently ignored after running claw setup
• Also remove the top-level model key when clearing provider settings

Before

{
  "provider": {
    "kind": "openai",
    "apiKey": "...",
    "baseUrl": "...",
    "model": "glm-5.1-fast"  ← ignored by parser
  }
}

After

{
  "model": "glm-5.1-fast",   ← read correctly by parser
  "provider": {
    "kind": "openai",
    "apiKey": "...",
    "baseUrl": "..."
  }
}

Test plan

• cargo build --release — clean build
• Run claw setup, select a provider and model, verify model appears at top level in ~/.claw/settings.json
• Run claw /status and verify the model is reflected
• Run clear_user_provider_settings path and verify both provider and top-level model are removed

💘 Generated with Crush

[TheArchitectit]

Fix added to #2810 instead