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/.gitignore

@@ -1,4 +1,8 @@
-dist/
+frontend/dist/
+frontend/dist-desketop/
+frontend/.env.desktop
 data/
+webui-service.env
 *.db
 webui-settings.json.migrated
+.webui.pid