* 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.
