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/hooks/use-command-center.ts

@@ -332,7 +332,7 @@ export function useCommandCenter() {
         if (currentItems.length === 0) return;
         event.preventDefault();
         const index = Math.max(0, Math.min(activeIndexRef.current, currentItems.length - 1));
-        handleSelectItemRef.current(currentItems[index]!);
+        handleSelectItemRef.current(currentItems[index]);
         return;
       }