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)

.gitignore

@@ -31,6 +31,3 @@ test-result/
 # Local configuration (if you want to keep local overrides)
 local.nix
 flake.lock.bak
-
-# Documentation
-docs/