opencode: make the rules option also accept a path

modules/programs/opencode.nix

@@ -45,8 +45,15 @@ in
     };
 
     rules = lib.mkOption {
-      type = lib.types.lines;
+      type = lib.types.either lib.types.lines lib.types.path;
       default = "";
+      description = ''
+         You can provide global custom instructions to opencode.
+         The value is either:
+         - Inline content as a string
+         - A path to a file containing the content
+        This value is written to {file}`$XDG_CONFIG_HOME/opencode/AGENTS.md`.
+      '';
       example = lib.literalExpression ''
         '''
           # TypeScript Project Rules
@@ -73,10 +80,6 @@ in
           Read the following file immediately as it's relevant to all workflows: @rules/general-guidelines.md.
         '''
       '';
-      description = ''
-        You can provide global custom instructions to opencode; this value is
-        written to {file}`$XDG_CONFIG_HOME/opencode/AGENTS.md`.
-      '';
     };
 
     commands = lib.mkOption {
@@ -162,9 +165,14 @@ in
         );
       };
 
-      "opencode/AGENTS.md" = mkIf (cfg.rules != "") {
-        text = cfg.rules;
-      };
+      "opencode/AGENTS.md" = (
+        if lib.isPath cfg.rules then
+          { source = cfg.rules; }
+        else
+          (mkIf (cfg.rules != "") {
+            text = cfg.rules;
+          })
+      );
     }
     // lib.mapAttrs' (
       name: content: