Document the desktop tray applet and check --json

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/PACKAGING.md

@@ -103,6 +103,17 @@ Notes:
 - Validate package verification behavior on the target distro before enabling
   signed-package mismatch alerting.
 
+## Desktop Tray Applet (optional)
+
+The `[tray]` optional extra (`pip install 'enodia-sentinel[tray]'`) pulls in
+`pystray` and `Pillow` and provides the `enodia-sentinel-tray` console script.
+The extra is never required by the daemon; core runtime dependencies stay
+empty.
+
+`packaging/enodia-sentinel-tray.desktop` is an opt-in autostart entry. It is
+not installed to a system autostart path by default — desktop users copy it to
+`~/.config/autostart/` themselves.
+
 ## Package Hardening Checklist
 
 - Package version matches `pyproject.toml` and `enodia_sentinel.__version__`.