Add startup notification feature with user configuration option

package.json

@@ -141,6 +141,11 @@
           "type": "boolean", 
           "default": true,
           "description": "Enable visual flashing notifications when AI agents send messages"
+        },
+        "humanagent-mcp.notifications.showStartup": {
+          "type": "boolean",
+          "default": true,
+          "description": "Show startup notification when HumanAgent MCP extension is activated"
         }
       }
     }