fix(chat): cancel orphaned generation on client disconnect

Closing the QuickChat browser tab mid-response left the backend agent
running with no listener. The `activeGenerations` slot persisted and the
next message's freshly-opened CLI SessionManager raced against the
lingering agent on the same session file, so the model produced no
output for the new prompt.

- POST /chat/sessions/:id/messages now calls cancelGeneration on
  req.on("close") if the response hasn't ended yet, so disconnects stop
  the agent promptly.
- beginGeneration only aborts the prior controller; it no longer
  disposes the prior agent. Disposing pre-emptively could yank the CLI
  process out from under the new generation's session opened on the
  same file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/src/routes/register-chat-routes.ts

@@ -560,6 +560,15 @@ export function registerChatRoutes(ctx: ApiRoutesContext, deps: ChatRouteDeps):
       // Handle client disconnect
       req.on("close", () => {
         unsubscribe();
+        // If the response hasn't been ended (i.e. the generation was still
+        // streaming when the browser disconnected — tab close, navigation,
+        // network drop), cancel the agent so it doesn't keep running with
+        // nobody listening. Otherwise the next request for this session sees a
+        // stale `activeGenerations` entry and races against the lingering
+        // agent for the same CLI session file.
+        if (!res.writableEnded) {
+          chatManager.cancelGeneration(sessionId);
+        }
       });
 
       // Send heartbeat every 30s to keep connection alive