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)

.env.vllm

@@ -0,0 +1,13 @@
+# vllm server configuration
+# Used by: nix run .#vllm-start-llm  and  nix run .#vllm-start-embed
+
+# Force CPU backend — no CUDA/ROCm GPU on this machine
+VLLM_TARGET_DEVICE=cpu
+
+VLLM_LLM_MODEL=Qwen/Qwen3-0.6B
+VLLM_LLM_PORT=8000
+# VLLM_LLM_EXTRA_ARGS=--dtype bfloat16 --max-model-len 4096
+
+VLLM_EMBED_MODEL=Qwen/Qwen3-Embedding-0.6B
+VLLM_EMBED_PORT=8001
+# VLLM_EMBED_EXTRA_ARGS=--dtype bfloat16