Remove unnecessary type assertions across codebase

Add oxlint-tsgolint and configure typescript/no-unnecessary-type-assertion
to flag redundant `!` and `as Foo` casts. Type-aware mode is left off by
default to keep `npm run lint` fast; the rule sits configured for when we
turn type-aware on intentionally. Auto-fix removed ~283 redundant casts;
two manual touch-ups: a real tsgolint false positive in split-container.tsx
and a stale ChildProcess import after a double-cast collapsed.

packages/app/src/components/project-picker-modal.tsx

@@ -29,7 +29,7 @@ interface PathRowProps {
 function PathRow({ path, active, onSelect }: PathRowProps) {
   const { theme } = useUnistyles();
   const handlePress = useCallback(() => {
-    void onSelect(path);
+    onSelect(path);
   }, [onSelect, path]);
   const pressableStyle = useCallback(
     ({ hovered = false, pressed }: PressableStateCallbackType & { hovered?: boolean }) => [
@@ -172,7 +172,7 @@ export function ProjectPickerModal() {
       if (key === "Enter") {
         event.preventDefault();
         if (options.length > 0 && activeIndex < options.length) {
-          void handleSelectPath(options[activeIndex]!);
+          void handleSelectPath(options[activeIndex]);
         } else if (query.trim()) {
           handleSubmitCustom();
         }