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/services/models-config.ts

@@ -0,0 +1,24 @@
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { dirname } from "node:path";
+import type { WebUiPaths } from "../config/paths.ts";
+
+export function readModelsConfig(paths: WebUiPaths): string {
+	if (!existsSync(paths.modelsConfigFile)) return "{\n  \"providers\": {}\n}\n";
+	return readFileSync(paths.modelsConfigFile, "utf8");
+}
+
+export function writeModelsConfig(paths: WebUiPaths, content: string): void {
+	let parsed: unknown;
+	try {
+		parsed = JSON.parse(content);
+	} catch (err) {
+		const message = err instanceof Error ? err.message : String(err);
+		throw new Error(`models.json 不是有效的 JSON: ${message}`);
+	}
+	if (parsed === null || typeof parsed !== "object" || Array.isArray(parsed)) {
+		throw new Error("models.json 根节点必须是 JSON 对象");
+	}
+	const formatted = `${JSON.stringify(parsed, null, 2)}\n`;
+	mkdirSync(dirname(paths.modelsConfigFile), { recursive: true });
+	writeFileSync(paths.modelsConfigFile, formatted, "utf8");
+}