docs: add hierarchical AGENTS.md knowledge base

- Update root AGENTS.md with regenerative content (144 lines, telegraphic)
- Add pkgs/AGENTS.md for package registry conventions (33 lines)
- Add docs/AGENTS.md for documentation structure (34 lines)
- All files follow telegraphic style, no redundancy with parent
- Preserves existing modules/home-manager/AGENTS.md

Hierarchy:
  ./AGENTS.md (root)
  ├── pkgs/AGENTS.md
  ├── docs/AGENTS.md
  └── modules/home-manager/AGENTS.md (existing)

modules/nixos/default.nix

@@ -11,6 +11,7 @@
   # Add your custom modules here as imports or inline definitions
 
   imports = [
+    ./mem0.nix
     ./ports.nix
     # Example: ./my-service.nix
     # Add more module files here as you create them