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/helpers/new-workspace.ts

@@ -170,9 +170,12 @@ export async function openNewWorkspaceComposer(
 
 export async function clickNewWorkspaceButton(
   page: Page,
-  input: { projectKey: string; projectDisplayName: string },
+  input: { projectKey: string; projectDisplayName: string; prompt?: string },
 ): Promise<void> {
   await openNewWorkspaceComposer(page, input);
+  const composer = page.getByRole("textbox", { name: "Message agent..." });
+  await expect(composer).toBeVisible({ timeout: 30_000 });
+  await composer.fill(input.prompt ?? "Hello from e2e");
   const createButton = page
     .getByTestId("message-input-root")
     .getByRole("button", { name: "Create" });