fix(agent-manager): left-align add-session button next to tabs (#519)

The + button was pushed to the far right alongside the terminal button
because the tab scroll area used flex:1 to fill all available space.
Replace flex:1 with flex-shrink:1 so the scroll area only sizes to its
content, keeping the + button immediately after the last tab.

packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css

@@ -333,9 +333,10 @@
   position: relative;
   display: flex;
   min-width: 0;
-  flex: 1;
+  flex-shrink: 1;
   align-items: stretch;
   height: 100%;
+  overflow: hidden;
 }
 
 /* Fade indicators for overflow */