feat: start agent immediately in background after user-mode install

After installing cron, the agent now starts right away via
nohup ... & disown instead of waiting for the next boot.

install/agent-install.sh

@@ -135,6 +135,9 @@ else
         echo "WARNING: Could not install crontab."
         echo "  Run: $AGENT_BIN &"
     }
+    echo "  Starting agent in background..."
+    nohup "$AGENT_BIN" > "$LOG_DIR/agent.log" 2>&1 & disown
+    echo "  Agent started (PID $!)"
 fi
 
 echo ""