fix(control-ui): create sessions for typed /new

Route typed Control UI `/new` through the dashboard session create-and-switch flow used by the New Chat button.

Keep typed `/reset` as the explicit in-place gateway reset path, and document the Control UI slash-command boundary.

Fixes #69599.

docs/tools/slash-commands.md

@@ -125,6 +125,7 @@ Current source-of-truth:
 <AccordionGroup>
   <Accordion title="Sessions and runs">
     - `/new [model]` starts a new session; `/reset` is the reset alias.
+    - Control UI intercepts typed `/new` to create and switch to a fresh dashboard session; typed `/reset` still runs the Gateway's in-place reset.
     - `/reset soft [message]` keeps the current transcript, drops reused CLI backend session ids, and reruns startup/system-prompt loading in-place.
     - `/compact [instructions]` compacts the session context. See [Compaction](/concepts/compaction).
     - `/stop` aborts the current run.