Indicate and jump to tmux panes waiting for attention

Show a bar indicator whenever tmux has flagged a window, the same state
that highlights the tab, and jump to it on click or with Super + Ctrl + J.
Tmux hooks push the state to the shell, so nothing polls while no pane is
waiting.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

shell/plugins/bar/widgets/Indicators.manifest.json

@@ -36,6 +36,11 @@
             "label": "Reminder",
             "description": "Queued reminder status"
           },
+          {
+            "value": "TmuxAlert",
+            "label": "Tmux alert",
+            "description": "Tmux windows waiting for attention"
+          },
           {
             "value": "NightLight",
             "label": "Night light",