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+---
+name: install-packages
+description: Install packages on this Nix-managed system - never use brew/apt/pacman/etc., always update Nix configuration or use nix run
+---
+
+## What I do
+
+- Guide proper package installation on Nix-managed systems
+- Prevent use of imperative package managers (brew, apt, pacman, etc.)
+- Ensure reproducible system configuration
+
+## When to use me
+
+- When you need to install a package
+- When a command is not found
+- When setting up development tools or dependencies
+- Before running ANY `brew`, `apt`, or similar commands
+
+## Critical rules
+
+- NEVER run these commands: `brew`, `apt`, `apt-get`, `pacman`, `yum`, `dnf`
+- Any command that imperatively changes global system state
+
+## Ephemeral usage for one-off commands
+
+Use `nix run` for temporary/one-time package usage:
+
+```sh
+nix run nixpkgs#<package> -- <args>
+```
+
+Example:
+```sh
+nix run nixpkgs#cowsay -- "Hello"
+```
+
+## Permanent installation
+
+Per-project:
+
+1. Check the development shell configuration in `flake.nix`
+2. Add the package
+3. Reload the development shell
+
+Globally:
+
+1. Determine the OS configuration project:
+   - macOS: `~/Documents/macos-setup`
+   - NixOS: `~/Documents/nixos-setup`
+2. Find the right nix file to add the package:
+   - Explore the project structure (varies per project)
+   - Common locations: `configuration.nix`, `default.nix`
+   - Consider: system vs user package, stable vs unstable channel
+   - If unsure where to add it, ask the user
+3. Add the package to the appropriate packages list
+4. Apply changes:
+   - `make` in the OS configuration project directory
+   - It may require sudo password or TouchID authentication from the user