feat(pi): preserve provider/model fields and sync Pi default provider/model on switch

- Fix stale configRef in useConfig so saves include the latest state.
- Preserve Pi models.json fields (headers, compat, cost, maxTokens,
  thinkingLevelMap, authHeader, modelOverrides, provider name) across edits.
- Add Pi settings.json I/O and update defaultProvider/defaultModel when
  activating a provider so Pi actually picks up the switch.
- Add round-trip and settings serialization tests.

src/hooks/useConfig.ts

@@ -48,7 +48,11 @@ export function useConfig(agent: Agent): UseConfigReturn {
   }, [refresh]);
 
   const updateConfig = useCallback((updater: (config: Config) => Config) => {
-    setConfig((prev) => (prev ? updater(prev) : prev));
+    setConfig((prev) => {
+      const next = prev ? updater(prev) : prev;
+      configRef.current = next;
+      return next;
+    });
     setDirty(true);
   }, []);