home/gui: Guard newer Codex settings option

Only set `programs.codex.mutableSettings` when Home Manager exposes it.
This fixes evaluation on 26.05, including boxes with the GUI disabled.
Record the removal TODO and guidance for future compatibility guards.

AGENTS.md

@@ -116,6 +116,11 @@ Four pkgs sets are threaded everywhere as `pkgsFlakes` / `pkgs'` (and `hmFlakes`
 channel via `nixpkgs` / `home-manager` / `hmNixpkgs` options (e.g. `nixpkgs = "mine-stable"`).
 Modules receive `pkgs'` = an attrset of all channels for the current system.
 
+For options absent from older channels, omit the attribute with `optionalAttrs` guarded by
+the module's `options` argument. `mkIf false` still checks option existence. Add a `TODO`
+comment explaining which older release needs the guard and which release upgrade allows
+its removal, matching the existing release-gated TODOs.
+
 ### `lib.my` and the `my` option namespace
 `lib/default.nix` extends `lib` with a `my` attrset (helpers like `mkOpt'`, `mkBoolOpt'`,
 `mkDefault'`, `inlineModule'`, `mkDefaultSystemsPkgs`, `homeStateVersion`). It also pulls in:

home-manager/modules/gui/default.nix

@@ -1,6 +1,6 @@
-{ lib, pkgs', pkgs, inputs, config, ... }:
+{ lib, pkgs', pkgs, inputs, config, options, ... }:
 let
-  inherit (lib) genAttrs mkIf mkMerge mkForce mapAttrs mkOptionDefault mkDefault optional;
+  inherit (lib) genAttrs mkIf mkMerge mkForce mapAttrs mkOptionDefault mkDefault optional optionalAttrs;
   inherit (lib.my) mkOpt' mkBoolOpt';
   inherit (lib.my.c) pubDomain;
 
@@ -187,10 +187,10 @@ in
             };
           };
 
+          # TODO: Remove mutableSettings guard when Home Manager 26.11 releases; 26.05 lacks it.
           codex = {
             enable = true;
             package = inputs.llm-agents.packages.${pkgs.stdenv.hostPlatform.system}.codex;
-            mutableSettings = true;
             settings.sandbox_workspace_write.network_access = true;
             settings.tui = {
               status_line = [
@@ -206,6 +206,8 @@ in
               ];
               status_line_use_colors = true;
             };
+          } // optionalAttrs (options.programs.codex ? mutableSettings) {
+            mutableSettings = true;
           };
 
           claude-code = {