refactor: reorganize flake structure and consolidate user config

Directory structure:
- Move from src/ to root level (system/, user/)
- Remove unused machines (workstation, vm, laptop)

User configuration:
- Add user/home.nix for shared defaults (pass, essentials, default modules)
- Centralize user options in user/default.nix
- Move submodules to consistent paths (bash/bash, git/git, neovim/nvim, vim/vim)

Module reorganization:
- Flatten nested module structures (remove /modules/ subdirs)
- Split CLI vs GUI tools (dev/ for CLI, gui/dev/ for GUI)
- Move neovim/vim to top-level modules (not under utils/)
- Remove security.enable - pass now in user/home.nix
- Remove utils.enable - essentials now in user/home.nix
- Add security/yubikey module with yubikey-manager, age-plugin-yubikey
- Move pcb, design to gui/dev/
- Replace penpot docker wrapper with nixpkgs penpot-desktop
- Remove i3 config
- Remove deprecated wsl.nativeSystemd option

GUI improvements:
- Browser-focused mimeApps in gui/default.nix
- Each WM handles its own auto-start via profileExtra

Cleanup:
- Update README with new structure
- Update justfile paths and valid systems
- Fix submodule paths in .gitmodules

user/keys/ssh/work.pub.key

@@ -0,0 +1 @@
+ssh-rsa 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