Add CLAUDE.md with project context and nix eval guidance

https://claude.ai/code/session_01PwAXuaoJx7qD5FhVLsn7Sn

CLAUDE.md

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+# FredOS NixOS Configuration
+
+This is a NixOS flake-based configuration for multiple hosts:
+- **FredOS-Gaming** — gaming desktop
+- **FredOS-Mediaserver** — home media server
+- **FredOS-Macbook** — MacBook laptop
+
+## Structure
+
+- `flake.nix` — flake inputs/outputs; all hosts use `nixpkgs` unstable
+- `common.nix` — shared configuration across all hosts
+- `hosts/` — per-host NixOS configuration modules
+- `hosts/hardware/` — hardware-specific configuration
+- `home-manager/` — Home Manager configuration (via NixOS module)
+- `services/` — modular service definitions imported by hosts
+- `settings/` — shared settings/variables
+
+## Code Evaluation
+
+Always validate Nix expressions with `nix eval` before committing. For example:
+
+```bash
+# Evaluate a specific attribute to check for syntax/type errors
+nix eval .#nixosConfigurations.FredOS-Gaming.config.system.stateVersion
+
+# Evaluate the full flake outputs to catch top-level errors
+nix eval .#nixosConfigurations --apply builtins.attrNames
+```
+
+Use `nix flake check` for a broader check of the flake.