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/sidebar-workspace-list.test.tsx

@@ -351,7 +351,7 @@ describe("sidebar workspace render isolation", () => {
     act(() => {
       useSessionStore.getState().mergeWorkspaces(SERVER_ID, [
         {
-          ...createWorkspaces()[1]!,
+          ...createWorkspaces()[1],
           status: "running",
         },
       ]);