fix(s3): guard switch_applications against missing wmctrl window match

[starm2010]

Problem

The UBUNTU_APP_SETUP template used by agent.switch_applications() references window_id without initializing it. When difflib.get_close_matches finds no match (target app not open, or no window with a similar title), exec() raises:

NameError: name 'window_id' is not defined

Because cli_app.run_agent() executes the generated code with no error handling, a single missing-window switch kills the entire agent process mid-task.

Solution

Initialize window_id = None before the match loop and guard the two wmctrl calls with if window_id:. When no window matches, the action becomes a no-op instead of a crash — the agent sees an unchanged screen on the next step and can adapt (open the app another way, or honestly report it is not open).

Testing

• RED: executed the real template from grounding.py with an app name matching no window → NameError kills the process (reproducible).
• GREEN: same scenario with the guard → template completes cleanly.
• E2E: task "switch to Calculator" with Calculator not open — before: process dead at step 1 (42s). After: agent continues, reports the app is not open, and completes the task with an honest failure report.