chore: drop omarchy pkg-* compatibility shims

These existed only to ease muscle-memory from Arch-based distros and add
no behavior on a Nix-native system:

- nomarchy-pkg-install — alias for nomarchy-pkg-add
- nomarchy-pkg-drop    — alias for nomarchy-pkg-remove
- nomarchy-pkg-aur-add — stub that only printed "AUR doesn't exist on NixOS"

The single code caller (nomarchy-voxtype-remove) was removed in the prior
commit, so the shims have no remaining references. SKILL.md updated to use
nomarchy-pkg-add and to point at `nix search nixpkgs` instead of the AUR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/home/config/nomarchy-skill/SKILL.md

@@ -109,7 +109,7 @@ cat $(which nomarchy-theme-set)
 | `nomarchy-install-*` | Install optional software | `nomarchy-install-docker-dbs` |
 | `nomarchy-launch-*` | Launch apps | `nomarchy-launch-browser` |
 | `nomarchy-cmd-*` | System commands | `nomarchy-cmd-screenshot` |
-| `nomarchy-pkg-*` | Package management | `nomarchy-pkg-install <pkg>` |
+| `nomarchy-pkg-*` | Package management | `nomarchy-pkg-add <pkg>` |
 | `nomarchy-setup-*` | Initial setup tasks | `nomarchy-setup-fingerprint` |
 | `nomarchy-update-*` | System updates | `nomarchy-update` |
 
@@ -377,7 +377,7 @@ When user requests system changes:
 2. **Is it a config edit?** Edit in `~/.config/`, never `~/.local/share/nomarchy/`
 3. **Is it a theme customization?** Create a NEW custom theme directory
 4. **Is it automation?** Use hooks in `~/.config/nomarchy/hooks/`
-5. **Is it a package install?** Use `nomarchy-pkg-add` (or `nomarchy-pkg-aur-add` for AUR-only packages)
+5. **Is it a package install?** Use `nomarchy-pkg-add` (adds to `user-packages.json`; rebuild applies it). The AUR does not exist on NixOS — search nixpkgs with `nix search nixpkgs <name>`.
 6. **Unsure if command exists?** Search with `compgen -c | grep nomarchy`
 
 ## Out of Scope