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/combined-model-selector.tsx

@@ -622,7 +622,7 @@ export function CombinedModelSelector({
   const singleProviderView = useMemo<SelectorView | null>(() => {
     const providers = Array.from(allProviderModels.keys());
     if (providers.length !== 1) return null;
-    const providerId = providers[0]!;
+    const providerId = providers[0];
     const label = resolveProviderLabel(providerDefinitions, providerId);
     return { kind: "provider", providerId, providerLabel: label };
   }, [allProviderModels, providerDefinitions]);
@@ -655,7 +655,7 @@ export function CombinedModelSelector({
 
   const handleSelect = useCallback(
     (provider: string, modelId: string) => {
-      onSelect(provider as AgentProvider, modelId);
+      onSelect(provider, modelId);
       setIsOpen(false);
       setSearchQuery("");
     },