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/system-prompt.ts

@@ -0,0 +1,13 @@
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { dirname } from "node:path";
+import type { WebUiPaths } from "../config/paths.ts";
+
+export function readSystemPrompt(paths: WebUiPaths): string {
+	if (!existsSync(paths.systemPromptFile)) return "";
+	return readFileSync(paths.systemPromptFile, "utf8");
+}
+
+export function writeSystemPrompt(paths: WebUiPaths, content: string): void {
+	mkdirSync(dirname(paths.systemPromptFile), { recursive: true });
+	writeFileSync(paths.systemPromptFile, content, "utf8");
+}