Rename express/ to backend/ and update references

Update paths in sync.sh, master/main.go, and CLAUDE.md to reflect
the directory rename.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/show-config.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+set -e
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+check_dir="$DIR"
+
+source "$check_dir"/../framework/shims/common
+source "$check_dir"/../framework/shims/node.common
+
+$ROOT/cmd pnpm tsc --showConfig