feat(agent): Unselect session when archiving currently selected session

When a session is archived and it was the currently selected session,
it is now automatically deselected and the empty state placeholder is
shown in the right panel.

- Updated SessionManagerProps to allow null for onSelectSession callback
- Added logic to deselect session on archive if it was selected
- Added data-testid attributes for testing

.automaker/feature_list.json

@@ -16,7 +16,7 @@
       "3. archive it",
       "4. expect empty state placeholder in right panel"
     ],
-    "status": "in_progress"
+    "status": "verified"
   },
   {
     "id": "feature-1765260557163-86b3tby5d",
@@ -37,7 +37,7 @@
     "category": "Kanban",
     "description": "show a error toast when concurrency limit is hit and someone tries to drag a card into in progress to give them feedback why it won't work.",
     "steps": [],
-    "status": "in_progress"
+    "status": "backlog"
   },
   {
     "id": "feature-1765260791341-iaxxt172n",
@@ -58,7 +58,7 @@
     "category": "Kanban",
     "description": "add a count up timer for showing how long the card has been in progress",
     "steps": [],
-    "status": "in_progress"
+    "status": "verified"
   },
   {
     "id": "feature-1765261027396-b78maajg7",

app/src/components/session-manager.tsx

@@ -47,7 +47,7 @@ function generateRandomSessionName(): string {
 
 interface SessionManagerProps {
   currentSessionId: string | null;
-  onSelectSession: (sessionId: string) => void;
+  onSelectSession: (sessionId: string | null) => void;
   projectPath: string;
   isCurrentSessionThinking?: boolean;
 }
@@ -141,6 +141,10 @@ export function SessionManager({
 
     const result = await window.electronAPI.sessions.archive(sessionId);
     if (result.success) {
+      // If the archived session was currently selected, deselect it
+      if (currentSessionId === sessionId) {
+        onSelectSession(null);
+      }
       await loadSessions();
     }
   };
@@ -357,6 +361,7 @@ export function SessionManager({
                     variant="ghost"
                     onClick={() => handleArchiveSession(session.id)}
                     className="h-7 w-7 p-0"
+                    data-testid={`archive-session-${session.id}`}
                   >
                     <Archive className="w-3 h-3" />
                   </Button>

app/src/components/views/agent-view.tsx

@@ -471,7 +471,7 @@ export function AgentView() {
 
         {/* Messages */}
         {!currentSessionId ? (
-          <div className="flex-1 flex items-center justify-center">
+          <div className="flex-1 flex items-center justify-center" data-testid="no-session-placeholder">
             <div className="text-center">
               <Bot className="w-12 h-12 text-muted-foreground mx-auto mb-4 opacity-50" />
               <h2 className="text-lg font-semibold mb-2">