Ship uninstaller in agent bundle and harden tray launch

The site-agent bundle now includes uninstall-shopdbagent.ps1 so operators can cleanly remove an install without hunting for the repo script. The packaging route reads it from scripts/, which was not mounted into the api container, so add a read-only ./scripts:/app/scripts mount (recreate the container to apply).

install.ps1 now registers the tray to autostart for every user via the HKLM Run value ShopDbEndpointTray, adds an All-Users 'ShopDB Agent' Start Menu shortcut for manual relaunch, and launches the tray once post-install. The tray enforces a single instance via a Global named mutex so repeated launches no longer stack duplicate icons. The uninstaller removes the Start Menu shortcut alongside the existing Run key and credential cleanup.

agent/ShopDbEndpointAgent.Tray/Program.cs

@@ -1,6 +1,8 @@
+using System.Threading;
+
 namespace ShopDbEndpointAgent.Tray;
 
-// Interactive user-session tray app (SCAFFOLD). Separate process from the
+// Interactive user-session tray app (SCAFFOLD).
 // headless Windows Service: the service owns telemetry, this owns the operator
 // surface - a system-tray icon and the 5-click-max ServiceNow ticket flow
 // (DESIGN-DECISIONS-agent.md section 6). This iteration wires the tray and a
@@ -8,9 +10,23 @@ namespace ShopDbEndpointAgent.Tray;
 // is deferred until that decision is made server-side.
 internal static class Program
 {
+    // Machine-wide single-instance guard. The installer starts the tray from an
+    // HKLM Run value for every user at logon, and an operator can also launch it
+    // from the Start Menu shortcut, so a second copy is easy to spawn. A named
+    // mutex in the Global namespace keeps exactly one tray per session: a later
+    // launch that cannot own the mutex exits immediately instead of stacking a
+    // duplicate icon. Global\ scopes across sessions on the same machine.
+    private const string SingleInstanceMutexName = @"Global\ShopDbEndpointAgentTray";
+
     [STAThread]
     private static void Main()
     {
+        using var singleInstance = new Mutex(true, SingleInstanceMutexName, out bool owned);
+        if (!owned)
+        {
+            return;
+        }
+
         ApplicationConfiguration.Initialize();
         Application.Run(new TrayContext());
     }