feat: add lightrag-mcp MCP server + agent tooling

- Add AGENTS.md with repo guidelines
- Add lightrag-mcp: FastMCP server exposing insert_documents() + query_documents()
  to LLM agents via stdio transport, talks to LightRAG REST API
- Add scripts/patch-vllm-cpu.py for CPU inference patching
- Add .env.vllm for vLLM configuration
- Update flake.nix with expanded dev shell
- Update .env.lightrag
- Remove CLAUDE.md (replaced by AGENTS.md)

scripts/patch-vllm-cpu.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+"""
+Patch vllm's cpu_platform_plugin to respect VLLM_TARGET_DEVICE=cpu.
+
+The upstream CUDA build only activates the CPU platform on macOS or when
+the version string contains "cpu" (source builds). This patch adds a third
+condition: if VLLM_TARGET_DEVICE=cpu is set in the environment.
+
+Run after every `uv pip install vllm` — idempotent.
+"""
+import pathlib
+import sys
+
+venv = pathlib.Path(__file__).parent.parent / "vllm" / ".venv"
+target = venv / "lib" / "python3.12" / "site-packages" / "vllm" / "platforms" / "__init__.py"
+
+if not target.exists():
+    print(f"vllm not installed at {target}, skipping patch")
+    sys.exit(0)
+
+content = target.read_text()
+
+if "VLLM_TARGET_DEVICE" in content:
+    print("patch already applied")
+    sys.exit(0)
+
+old = '''\
+        if not is_cpu:
+            import sys
+
+            is_cpu = sys.platform.startswith("darwin")
+            if is_cpu:
+                logger.debug(
+                    "Confirmed CPU platform is available because the machine is MacOS."
+                )'''
+
+new = old + '''
+
+        if not is_cpu:
+            is_cpu = os.environ.get("VLLM_TARGET_DEVICE", "").lower() == "cpu"
+            if is_cpu:
+                logger.debug(
+                    "Confirmed CPU platform is available because VLLM_TARGET_DEVICE=cpu."
+                )'''
+
+if old not in content:
+    print("ERROR: patch target not found — vllm version may have changed", file=sys.stderr)
+    sys.exit(1)
+
+target.write_text(content.replace(old, new, 1))
+print("patched cpu_platform_plugin to respect VLLM_TARGET_DEVICE=cpu")