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/contexts/voice-context.tsx

@@ -140,7 +140,7 @@ export function VoiceProvider({ children }: VoiceProviderProps) {
     runtimeRef.current = runtime;
   }
 
-  const engine = engineRef.current!;
+  const engine = engineRef.current;
   const runtime = runtimeRef.current!;
 
   useEffect(() => {