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/server/src/server/session.workspaces.test.ts

@@ -3809,7 +3809,7 @@ test("subscribed fetch_workspaces includes git enrichment in the initial snapsho
   session.workspaceRegistry.list = async () => [gitWorkspace, directoryWorkspace];
   session.reconcileAndEmitWorkspaceUpdates = vi.fn(async () => {});
   session.describeWorkspaceRecord = vi.fn(
-    async (workspace: typeof gitWorkspace | typeof directoryWorkspace, project: unknown) => {
+    async (workspace: typeof gitWorkspace, project: unknown) => {
       if (workspace.workspaceId === gitWorkspace.workspaceId) {
         expect(project).toEqual(gitProject);
         return baselineGitDescriptor;