feat(app): create empty workspace without a prompt (#834)

* feat(app): create empty workspace from new-workspace composer

Pressing Create with an empty composer (no text, no attachments) on the
new-workspace screen now creates the workspace without an agent and
lands the user on the empty-state, where they can open a terminal or
start an agent on their own. Dispatching submit handler routes to a
named runCreateEmptyWorkspace sibling of runCreateChatAgent so empty
creation is a first-class action, not a hidden branch in the chat path.

* refactor(app): extract empty-workspace helpers into own module

Gives the empty-workspace dispatch a real home, shrinks the test mock
graph from ~20 modules to one.

* test(app): type prompt in new-workspace e2e to exercise chat path

Empty-submission now creates a bare workspace (no agent), so tests that
assert agent-tab/optimistic-draft behavior must type a prompt to take
the chat path.

packages/app/e2e/new-workspace.spec.ts

@@ -306,6 +306,10 @@ test.describe("New workspace flow", () => {
         projectDisplayName: openedProject.projectDisplayName,
       });
 
+      const composer = page.getByRole("textbox", { name: "Message agent..." });
+      await expect(composer).toBeVisible({ timeout: 30_000 });
+      await composer.fill("Hello from e2e");
+
       const createButton = page
         .getByTestId("message-input-root")
         .getByRole("button", { name: "Create" });