fix(host): drop in-flight design session when falling to chat path

Codex stop-time review caught a real cross-session leak in the intent
gate introduced by the previous commit. `launch_if_pending` clears
`current_chat` when the design branch is taken, but the chat branch
only cleared `current_chat` (for the unwired-agent error path) — it
never cleared `current_design`.

Symptom: if a design turn is in flight (worker pumping `apply` requests
+ `Progress` deltas into the trailing assistant bubble) and the user
sends a chat message, the new chat bubble begins streaming chat deltas
while the still-running design worker keeps acking apply requests on
the canonical doc and appending `Progress` strings to the same bubble.
The fix mirrors the design branch — when the chat path is taken (chat
intent, design intent without a Provider, or any unwired agent),
`*current_design = None` drops the worker's command receiver so its
next `apply` returns false and the turn ends.

`cargo test -p op-host-desktop` 105 passed. workspace fmt + clippy
clean.

crates/op-host-desktop/src/chat_session.rs

@@ -195,6 +195,13 @@ pub fn launch_if_pending(
         // the chat path. The assistant CLI will still answer the user
         // (most CLIs handle design verbs as chat).
     }
+    // Taking the chat path — drop any in-flight design turn so its
+    // worker's next `apply` returns false (channel dropped) and its
+    // `Progress` deltas stop streaming into this turn's fresh bubble
+    // (codex stop-gate: stale design session survived chat fallback,
+    // kept overwriting the new bubble content + applying ack'd
+    // EditorCommands long after the user moved on).
+    *current_design = None;
     let agent_idx = host.editor_state().editor_ui.chat_selected_agent;
     let Some(provider) = provider_for_agent(agent_idx) else {
         // Selected agent has no `ChatProvider` bridge yet (Codex /