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.test.ts

@@ -63,8 +63,8 @@ describe("combined model selector helpers", () => {
       }),
     ]);
 
-    expect(matchesSearch(rows[0]!, "claude")).toBe(true);
-    expect(matchesSearch(rows[1]!, "gpt-5.4")).toBe(true);
+    expect(matchesSearch(rows[0], "claude")).toBe(true);
+    expect(matchesSearch(rows[1], "gpt-5.4")).toBe(true);
   });
 
   it("keeps the selected trigger label model-only", () => {