update: README.md

AGENTS.md

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+# AGENTS.md - NixOS Configuration Repository
+
+## 1. Project Context
+This is a **NixOS Flake configuration** for a personal desktop system (hostname: `nix`, user: `pinj`).
+It relies on modern Nix features (Flakes, experimental commands) and a modular architecture.
+
+## 2. Build & Validation Commands
+
+### Core Workflows
+Use these commands to verify your changes. **Always run the test command before asking the user to switch.**
+
+```bash
+# 1. SYNTAX CHECK (Fastest)
+# Checks for basic syntax errors without evaluating the entire system
+nix-instantiate --parse configuration.nix
+
+# 2. FLAKE CHECK (Unit Tests)
+# Validates flake structure and evaluates outputs. Use this to ensure no regression.
+nix flake check
+
+# 3. DRY RUN (Integration Test)
+# Builds the system and tries to activate it in a VM or checks for conflicts 
+# without modifying the bootloader or switching the current system.
+sudo nixos-rebuild test --flake .#nixos
+```
+
+### Deployment
+Only run these when explicitly requested by the user:
+
+```bash
+# Build (create derivation but don't switch)
+sudo nixos-rebuild build --flake .#nixos
+
+# Switch (apply changes to live system)
+sudo nixos-rebuild switch --flake .#nixos
+```
+
+### Maintenance
+```bash
+# Update inputs (e.g. nixpkgs)
+nix flake update
+
+# Format all nix files
+nixfmt *.nix modules/*.nix
+```
+
+## 3. Code Style & Conventions
+
+### Visual Structure
+Maintain the project's visual hierarchy. Use decorative comments to separate major sections.
+
+```nix
+# ═══════════════════════════════════════════════════════════════
+# SECTION NAME (UPPERCASE)
+# ═══════════════════════════════════════════════════════════════
+```
+
+### Nix Formatting
+- **Indentation:** 2 spaces.
+- **Lists:** Use `with pkgs; [ ... ]` for package lists to reduce verbosity.
+- **Line Length:** Soft limit 100 chars, hard limit 120.
+- **Functions:** Use standard module arguments: `{ config, pkgs, lib, ... }:`.
+
+### Module Pattern
+All functionality should be modularized in `./modules/`.
+- **Filename:** `kebab-case.nix` (e.g., `gpu-amd.nix`).
+- **Structure:**
+  ```nix
+  { config, pkgs, lib, ... }: {
+    # Options
+    programs.foo.enable = true;
+
+    # Packages
+    environment.systemPackages = with pkgs; [
+      foo-pkg
+    ];
+  }
+  ```
+
+### Naming & Variables
+- **Inputs:** Reference flake inputs via `inputs.name` (passed via `specialArgs`).
+- **Packages:** Use `pkgs.packageName`.
+- **User:** The primary user `pinj` is defined in `configuration.nix`.
+- **Groups:** Add user to groups via `extraGroups` in specific modules (e.g., `docker` group in `dev.nix`).
+
+## 4. Agent Guidelines
+
+### ⚠️ Critical Safety Rules
+1.  **Secrets:** NEVER commit secrets (tokens, keys, passwords). Check `.gitignore`.
+2.  **Hardware Config:** Do NOT modify `hardware-configuration.nix` unless explicitly checking for new drives/mounts. It is auto-generated.
+3.  **State Version:** Do NOT change `system.stateVersion` ("26.05").
+
+### Navigation & Editing
+- **Absolute Paths:** When using tools, always resolve paths fully (e.g., `/home/pinj/nixos/modules/dev.nix`).
+- **Read First:** Always read `configuration.nix` to see active imports before creating new modules.
+- **Search:** Use `grep` to find where existing programs are configured (e.g., `grep -r "firefox" .`).
+
+### Troubleshooting Common Errors
+- **"Option does not exist":** You might be missing an import in `configuration.nix` or using a home-manager option in a system module.
+- **"Hash mismatch":** If updating a `fetchurl` or flake input, ensure hashes are updated.
+- **"ReadOnly":** You cannot edit files in the `/nix/store`. Only edit files in `/home/pinj/nixos`.
+
+## 5. Directory Structure
+
+| Path | Purpose |
+|------|---------|
+| `flake.nix` | Entry point, dependencies (inputs), and outputs. |
+| `configuration.nix` | System glue. Imports modules, sets defaults, defines user. |
+| `modules/` | Feature-specific configs (desktop, dev, gaming, etc.). |
+| `hardware-configuration.nix` | Hardware scan (do not edit manually). |