feat(omo): coloured tab markers and clickable Ghostty banners

omo-notify now drives three signals from omo's UserPromptSubmit, Stop
and Notification hooks:

- A sound through afplay, so Focus modes cannot mute it
- A Ghostty notification (OSC 777) written to the session's tmux client
  terminal, so clicking it opens the tab it came from. This replaces the
  osascript banner, which opened Script Editor's file picker on click
- omo-status markers in front of the tab title: red error, amber
  waiting for input, green finished, blue working. A tab shows its most
  urgent agent; focusing it clears red, amber and green and selects the
  pane that sent the banner

tmux puts @omo_status in the title and runs omo-status on client focus.

.tmux.conf

@@ -64,7 +64,10 @@ bind -r m resize-pane -Z
 
 # Push tmux window/session info to the terminal title
 set -g set-titles on
-set -g set-titles-string "#{session_name}"
+# omo-status puts a coloured marker (@omo_status) in front of the session name;
+# focusing the tab clears the attention markers.
+set -g set-titles-string "#{?@omo_status,#{@omo_status} ,}#{session_name}"
+set-hook -g client-focus-in 'run-shell -b "~/.local/bin/omo-status focus #{q:hook_client}"'
 
 # Set plugin manager path before loading plugins
 set-environment -g TMUX_PLUGIN_MANAGER_PATH "$HOME/.tmux/plugins"