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/components/message-item.tsx

@@ -59,9 +59,7 @@ export default function MessageItem(props: MessageItemProps) {
           </div>
         </Show>
 
-        <For each={props.message.parts}>
-          {(part) => <MessagePart part={part} key={part.id || `${part.type}-${Math.random()}`} />}
-        </For>
+        <For each={props.message.parts}>{(part) => <MessagePart part={part} />}</For>
       </div>
 
       <Show when={props.message.status === "sending"}>