feat: add Codex CLI compatibility layer

- flake.nix: devShell with yq-go for config generation
- generate-codex.sh: generates Codex agent TOML, AGENTS.md, and
  config.toml from Claude source files (idempotent, yq-powered)
- install.sh: optional Codex symlinks when ~/.codex exists (skills
  shared via ~/.agents/skills/, agents/config/AGENTS.md to ~/.codex/)
- .gitignore: exclude generated codex/ directory
- README.md: document Codex compatibility setup and model mapping

flake.nix

@@ -0,0 +1,14 @@
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  outputs = { nixpkgs, ... }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+    in {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell {
+          packages = [ pkgs.yq-go ];
+        };
+      });
+    };
+}