feat(sproutclaw): modularize webui, extensions, and agent config layout

Restructure local extensions into per-feature directories, split WebUI
into backend modules with slash commands and systemd support, and track
prompts/skills under .pi/agent for portable Gitea deployment.

Co-authored-by: Cursor <cursoragent@cursor.com>

.pi/agent/extensions/exit-command/index.ts

@@ -0,0 +1,17 @@
+/**
+ * Exit Command Extension
+ *
+ * 添加 /exit 命令用于退出 pi Agent
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("exit", {
+		description: "退出 pi Agent",
+		handler: async (_args, ctx) => {
+			ctx.ui.notify("正在退出...", "info");
+			ctx.shutdown();
+		},
+	});
+}