opencode: support directory-based configuration for commands, agents, and themes

The `commands`, `agents`, and `themes` options now accept either an
attribute set (existing behavior) or a path to a directory containing
multiple files. When a directory path is provided, it is symlinked to
the appropriate `$XDG_CONFIG_HOME/opencode/` subdirectory.

This change aligns with the existing `skills` option implementation and
provides a more convenient way to manage multiple configuration files
without needing to define each one individually in Nix.

tests/modules/programs/opencode/agents-inline.nix

@@ -12,7 +12,8 @@
         - Review for potential bugs and edge cases
         - Check for security vulnerabilities
         - Ensure code follows best practices
-        - Suggest improvements for readability and performance'';
+        - Suggest improvements for readability and performance
+      '';
       documentation = ''
         # Documentation Agent
 
@@ -23,7 +24,8 @@
         - Write clear, concise documentation
         - Include practical examples
         - Use proper formatting and structure
-        - Consider the target audience'';
+        - Consider the target audience
+      '';
     };
   };
   nmt.script = ''