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/file-explorer-pane.tsx

@@ -132,7 +132,7 @@ function TreeRowItem({
   );
 
   const handleCopy = useCallback(() => {
-    void onCopyPath(entry.path);
+    onCopyPath(entry.path);
   }, [onCopyPath, entry.path]);
 
   const handleDownload = useCallback(() => {