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/scripts/webui-run.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+: "${REPO_ROOT:?REPO_ROOT required}"
+: "${EXTENSION_DIR:?EXTENSION_DIR required}"
+: "${NODE_BIN:?NODE_BIN required}"
+: "${TSX_CLI:?TSX_CLI required}"
+: "${PI_CODING_AGENT_DIR:?PI_CODING_AGENT_DIR required}"
+PORT="${PORT:-19133}"
+cd "$REPO_ROOT"
+exec "$NODE_BIN" "$TSX_CLI" "$EXTENSION_DIR/backend/main.ts" --port "$PORT"