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/screens/workspace/workspace-screen.tsx

@@ -1541,7 +1541,7 @@ function WorkspaceScreenContent({
       if (!client || !workspaceDirectory) {
         throw new Error("Host is not connected");
       }
-      return (await client.getCheckoutStatus(workspaceDirectory)) as CheckoutStatusPayload;
+      return await client.getCheckoutStatus(workspaceDirectory);
     },
     staleTime: Infinity,
     refetchOnMount: false,