fix(desktop): make the zsh probe test legs visible and run them on CI

The zsh login-shell legs in remote-lifecycle.test.ts and
ssh-connection.test.ts silently returned when zsh was missing, and the
js-tests runner image ships no zsh, so the #111949 coverage never ran on
CI and a wrapper regression stayed green.

- js-tests.yml: install zsh on the Linux runner before the checks.
- Both legs now report vitest skips ('zsh not installed') instead of
  passing; the ssh-connection leg is its own test so the skip is visible.
- Docs: note the zsh degraded mode (no process-group kill for a hung
  probe's grandchildren) in the SSH connection guide.

.github/workflows/js-tests.yml

@@ -23,6 +23,13 @@ jobs:
           node-version: 26
           cache: npm
 
+      # The desktop SSH watchdog tests run their probe through a real zsh
+      # login shell (#111949). The ubuntu image ships no zsh and the tests
+      # skip without it, so install it here or those legs never run on CI.
+      - name: Install zsh
+        if: runner.os == 'Linux'
+        run: sudo apt-get install -y zsh
+
       - name: grab npm 12
         run: |
           # No-op once the bundled npm is already 12.x — saves ~5-15s/job and