fix: stay in project root after nix develop .#lightrag

Use subshells for cd to lightrag/ so user remains at repo root.
Add CLAUDE.md with purpose, architecture, and known issues.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.nix

@@ -53,18 +53,14 @@
             export UV_PROJECT_ENVIRONMENT="$VIRTUAL_ENV"
             export UV_PYTHON="${pkgs.python312}/bin/python3.12"
             export LD_LIBRARY_PATH="${pkgs.lib.makeLibraryPath [ pkgs.stdenv.cc.cc pkgs.zlib ]}:$LD_LIBRARY_PATH"
-            cd "$RAGS_ROOT/lightrag"
 
             echo "Syncing lightrag venv..."
-            uv sync --extra api --extra offline-llm --quiet
-            source .venv/bin/activate
+            (cd "$RAGS_ROOT/lightrag" && uv sync --extra api --extra offline-llm --quiet)
+            source "$RAGS_ROOT/lightrag/.venv/bin/activate"
 
-            if [ ! -d lightrag/api/webui ]; then
+            if [ ! -d "$RAGS_ROOT/lightrag/lightrag/api/webui" ]; then
               echo "Building frontend (first time, ~1min)..."
-              cd lightrag_webui
-              bun install --frozen-lockfile --silent
-              bun run build:bun
-              cd ..
+              (cd "$RAGS_ROOT/lightrag/lightrag_webui" && bun install --frozen-lockfile --silent && bun run build:bun)
             fi
 
             if [ -f "$RAGS_ROOT/.env.lightrag" ]; then