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)

flake.nix

@@ -58,6 +58,7 @@
       default = ./modules/nixos;
       # Individual modules for selective imports
       ports = ./modules/nixos/ports.nix;
+      mem0 = ./modules/nixos/mem0.nix;
     };
 
     # Home Manager modules - for user-level configuration