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-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([