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,

packages/app/src/screens/workspace/workspace-tab-layout.test.ts

@@ -68,7 +68,7 @@ describe("computeWorkspaceTabLayout", () => {
     expect(result.closeButtonPolicy).toBe("all");
     expect(result.requiresHorizontalScrollFallback).toBe(false);
     expect(result.items.map((item) => item.width)).toEqual([60, 60, 60, 60]);
-    expect(result.items.every((item) => item.showLabel === false)).toBe(true);
+    expect(result.items.every((item) => !item.showLabel)).toBe(true);
   });
 
   it("allows horizontal scroll only when icon-only tabs still cannot fit", () => {
@@ -81,7 +81,7 @@ describe("computeWorkspaceTabLayout", () => {
     expect(result.closeButtonPolicy).toBe("all");
     expect(result.requiresHorizontalScrollFallback).toBe(true);
     expect(result.items.map((item) => item.width)).toEqual([60, 60, 60, 60]);
-    expect(result.items.every((item) => item.showLabel === false)).toBe(true);
+    expect(result.items.every((item) => !item.showLabel)).toBe(true);
   });
 
   it("returns empty layout details when there are no tabs", () => {

packages/app/src/screens/workspace/workspace-tab-model.test.ts

@@ -23,7 +23,7 @@ describe("deriveWorkspaceTabModel", () => {
     ];
 
     const model = deriveWorkspaceTabModel({
-      tabs: [uiTabs[0]!, uiTabs[2]!, uiTabs[1]!],
+      tabs: [uiTabs[0], uiTabs[2], uiTabs[1]],
     });
 
     expect(model.tabs.map((tab) => tab.descriptor.tabId)).toEqual([