refactor: update llm model configurations and add AI agent guidelines

- Update nvim to use qwen3-coder-next-80b-instruct model
- Add AGENTS.md with AI agent best practices for timeout and file writing
- Update pi config to include agent guidelines
- Refactor llama-swap: remove old models, update quantizations, add tensor splits,
  remove GGML_CUDA_ENABLE_UNIFIED_MEMORY flags, and simplify configuration

modules/home/programs/terminal/pi/default.nix

@@ -37,6 +37,9 @@ in
           };
         };
       };
+      ".pi/agent/AGENTS.md" = {
+        source = ./config/AGENTS.md;
+      };
       ".pi/agent/skills" = {
         source = ./config/skills;
         recursive = true;