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/webui/backend/main.ts

@@ -0,0 +1,83 @@
+#!/usr/bin/env node
+
+/**
+ * pi-mono WebUI Server
+ *
+ * 被 webui 扩展启动，作为独立子进程运行。
+ * 内嵌 HTTP 服务 + pi RPC 子进程，提供浏览器聊天界面。
+ *
+ * 用法（由扩展自动调用）：
+ *   tsx backend/main.ts --port 19133
+ *
+ * 前端静态文件位于 frontend/dist/ 下（Vite 构建产物）。
+ */
+
+import { existsSync, unlinkSync, writeFileSync } from "node:fs";
+import { parsePort } from "./config/cli.ts";
+import { createWebUiPaths } from "./config/paths.ts";
+import { closeWebuiDatabase, initWebuiDatabase } from "./db/index.ts";
+import { createWebUiServer } from "./http/server.ts";
+import { createPiClient } from "./rpc/pi-client.ts";
+import type { WebUiContext } from "./types/context.ts";
+
+const paths = createWebUiPaths();
+const port = parsePort();
+
+function writePidFile(): void {
+	try {
+		writeFileSync(paths.pidFile, String(process.pid), "utf8");
+	} catch {
+		/* ignore */
+	}
+}
+
+function clearPidFile(): void {
+	try {
+		if (existsSync(paths.pidFile)) unlinkSync(paths.pidFile);
+	} catch {
+		/* ignore */
+	}
+}
+
+function shutdown(exitCode = 0): never {
+	closeWebuiDatabase();
+	clearPidFile();
+	process.exit(exitCode);
+}
+
+const webuiDb = initWebuiDatabase(paths.extensionRoot);
+console.log(`[webui] 配置数据库: ${webuiDb.dbPath}`);
+
+process.on("SIGTERM", () => shutdown(0));
+process.on("SIGINT", () => shutdown(0));
+writePidFile();
+
+const piClient = createPiClient(paths, () => shutdown(1));
+
+const ctx: WebUiContext = {
+	config: { paths, port },
+	rpc: {
+		sendCmd: piClient.sendCmd,
+		submitPrompt: piClient.submitPrompt,
+		getRunSnapshot: piClient.getRunSnapshot,
+		connectSseClient: piClient.connectSseClient,
+		removeSseClient: piClient.removeSseClient,
+	},
+	db: webuiDb,
+};
+
+const server = createWebUiServer(ctx);
+
+server.on("error", (err: NodeJS.ErrnoException) => {
+	if (err.code === "EADDRINUSE") {
+		console.error(`[webui] 端口 ${port} 已被占用`);
+	} else {
+		console.error(`[webui] HTTP 服务启动失败: ${err.message}`);
+	}
+	process.exit(1);
+});
+
+server.listen(port, "0.0.0.0", () => {
+	console.log(`[webui] HTTP 服务已启动: http://localhost:${port}`);
+	console.log(`[webui] 局域网访问: http://smallmengya:${port}`);
+});