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/skills/github-gh-skill/.gitignore

@@ -0,0 +1,4 @@
+__pycache__/
+*.py[cod]
+.venv/
+venv/