Add configurable Claude Code model selection

Add allowArbitraryClaudeCodeModelSelection option to development role
to control whether model specifications are preserved in humanlayer
commands and agents. Defaults to false (strip models) for backward
compatibility. Enable on home-darwin-work to preserve model specs
for opus/sonnet selection.

home/home-darwin-work.nix

@@ -223,7 +223,10 @@ in
 
   home.roles = {
     base.enable = true;
-    development.enable = true;
+    development = {
+      enable = true;
+      allowArbitraryClaudeCodeModelSelection = true;
+    };
   };
 
   imports = [