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.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1775095191,
+        "narHash": "sha256-CsqRiYbgQyv01LS0NlC7shwzhDhjNDQSrhBX8VuD3nM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "106eb93cbb9d4e4726bf6bc367a3114f7ed6b32f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}