feat: add search parameter and auto-select to /model command

- /model <search> pre-filters selector or auto-selects on exact match
- Support provider/model syntax for disambiguation (e.g., /model openai/gpt-4)
- Auto-select logic moved to InteractiveMode to avoid selector UI flicker

Closes #587

.pi/extensions/prompt-url-widget.ts

@@ -80,7 +80,6 @@ export default function promptUrlWidgetExtension(pi: ExtensionAPI) {
 		if (!ctx.hasUI) return;
 		const match = extractPromptMatch(event.prompt);
 		if (!match) {
-			ctx.ui.setWidget("prompt-url", undefined);
 			return;
 		}