feat(agent): add nix development skills for Claude Code

Add comprehensive Nix skills to enhance Claude's assistance with:

Skills created:
- /nix: Main skill for flakes, darwin, NixOS, home-manager
- /nix-debug: Deep debugging (eval, repl, trace, debugger)
- /nix-flake: Flake evaluation and introspection
- /nix-module: Module development patterns
- /nix-service: launchd/systemd service management

Includes:
- Complete nix eval/repl command reference
- lib.debug.* function cookbook (traceVal, traceSeq, etc.)
- Interactive debugger (Nix 2.9+) guide
- Error diagnosis patterns with solutions
- Module system debugging techniques
- Platform-specific patterns (Darwin/Linux)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: r17x

.claude/skills/nix/SKILL.md

@@ -0,0 +1,168 @@
+---
+name: nix
+description: Nix flakes, nix-darwin, NixOS, and home-manager development assistance
+allowed-tools:
+  - Bash
+  - Read
+  - Grep
+  - Glob
+  - Edit
+  - Write
+---
+
+# Nix Development Skill
+
+## Evaluation & Debugging
+
+### Evaluate flake outputs
+```bash
+# List available outputs
+nix flake show
+
+# Evaluate specific attribute
+nix eval .#<attribute> --json
+
+# Darwin config options
+nix eval .#darwinConfigurations.<host>.options.<path>
+
+# Home-manager config
+nix eval .#homeConfigurations."<user>@<host>".config.<path>
+```
+
+### Debug evaluation errors
+```bash
+# Show full trace
+nix eval .#<attr> --show-trace
+
+# Check flake validity
+nix flake check
+
+# Instantiate without building
+nix-instantiate --eval -E '<expression>'
+```
+
+## Building & Rebuilding
+
+### Darwin (macOS)
+```bash
+darwin-rebuild switch --flake .
+darwin-rebuild switch --flake .#<hostname>
+
+# Rollback
+darwin-rebuild --list-generations
+darwin-rebuild switch --rollback
+```
+
+### NixOS
+```bash
+sudo nixos-rebuild switch --flake .#<hostname>
+sudo nixos-rebuild boot --flake .#<hostname>
+```
+
+### Home-manager standalone
+```bash
+home-manager switch --flake .#<user>@<host>
+```
+
+## Service Management (launchd/systemd)
+
+### macOS launchd
+```bash
+# List services
+launchctl list | grep -E "org.nixos|nix"
+
+# Service status
+launchctl print system/<label>
+launchctl print gui/$(id -u)/<label>
+
+# Control services
+launchctl kickstart [-k] <domain>/<label>
+launchctl kill SIGTERM <domain>/<label>
+```
+
+### Linux systemd
+```bash
+systemctl --user list-units --type=service
+systemctl --user status <service>
+systemctl --user restart <service>
+journalctl --user -u <service> -f
+```
+
+## Launchd Configuration Options
+
+For reducing CPU/IO priority in nix-darwin:
+```nix
+launchd.daemons.<name>.serviceConfig = {
+  RunAtLoad = false;        # Don't start at boot
+  KeepAlive = false;        # Don't auto-restart
+  Nice = 5;                 # Lower CPU priority (1-20)
+  ProcessType = "Background"; # Background scheduling
+  LowPriorityIO = true;     # Lower I/O priority
+  ThrottleInterval = 10;    # Min seconds between restarts
+};
+
+launchd.user.agents.<name>.serviceConfig = { /* same options */ };
+```
+
+## Common Patterns
+
+### Override with mkForce
+```nix
+# When upstream sets a value you need to override
+someOption = lib.mkForce false;
+```
+
+### Conditional by platform
+```nix
+# At Nix level (preferred)
+serviceCommands = if pkgs.stdenv.isDarwin
+  then import ./darwin.nix
+  else import ./linux.nix;
+
+# In module
+config = lib.mkIf pkgs.stdenv.isDarwin { ... };
+```
+
+### Module structure
+```nix
+{ lib, config, pkgs, ... }:
+let
+  cfg = config.myModule;
+in {
+  options.myModule = {
+    enable = lib.mkEnableOption "my module";
+  };
+
+  config = lib.mkIf cfg.enable {
+    # implementation
+  };
+}
+```
+
+## Flake Inputs Management
+
+```bash
+# Update all inputs
+nix flake update
+
+# Update specific input
+nix flake lock --update-input <input-name>
+
+# Show inputs
+nix flake metadata
+```
+
+## Troubleshooting
+
+### "infinite recursion" error
+- Check for circular dependencies in imports
+- Use `lib.mkDefault` or `lib.mkForce` to resolve conflicts
+
+### "attribute not found"
+- Verify the attribute path with `nix eval`
+- Check if module is properly imported
+
+### Service not starting
+- Check plist/unit file generation
+- Verify paths in ProgramArguments
+- Check logs for errors