test(desktop): stop the agent's terminal commands in the real app, with real shells and real tmux

An Electron suite for terminal cancel, which ships without a feature flag:
Stop, sign-out (by the web app's logout and by the session cookie going away),
switching Terminal off, the launch after an interrupted sign-out or a crash,
and untracked runs on tmux without pane options. It asserts what is running
and which panes exist, never which calls were made.

- Agent and user commands ignore SIGHUP, SIGINT and SIGTERM, so only Sim's own
  stop can end them and a stray stop would show; each launch gets its own tmux
  server, and every process name carries a per-run suffix.
- The fixture Sim and its helpers move to e2e/executor-sim.ts, shared with the
  background executor suite.
- The macOS E2E job installs tmux so the tmux scenarios run in CI; elsewhere
  they skip when tmux is missing.

Author: waleedlatif1

.github/workflows/desktop-e2e.yml

@@ -81,6 +81,10 @@ jobs:
         working-directory: apps/desktop
         run: bunx playwright install chromium
 
+      # The terminal-cancel suite drives a real tmux server; without tmux its tmux scenarios skip.
+      - name: Install tmux
+        run: brew install tmux
+
       - name: Run Playwright _electron smoke suite
         working-directory: apps/desktop
         run: bunx playwright test