Add agent and model selection with automatic defaults and subagent support

- Implement automatic agent/model selection for new sessions (first agent, prioritize Anthropic default)
- Extract and restore agent/model from last assistant message when resuming sessions
- Add subagent support: child sessions can select subagents, parent sessions cannot
- Display subagent badge in agent dropdown for identification
- Truncate agent descriptions to 50 characters in dropdown
- Improve model search to include full provider/model ID path
- Auto-select input text on focus for easy model search
- Add getDefaultModel() with priority: agent model → Anthropic → first provider

src/types/session.ts

@@ -22,12 +22,17 @@ export interface Agent {
   name: string
   description: string
   mode: string
+  model?: {
+    providerId: string
+    modelId: string
+  }
 }
 
 export interface Provider {
   id: string
   name: string
   models: Model[]
+  defaultModelId?: string
 }
 
 export interface Model {