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/commands-inline.nix

@@ -6,12 +6,14 @@
         # Update Changelog Command
 
         Update CHANGELOG.md with a new entry for the specified version.
-        Usage: /changelog [version] [change-type] [message]'';
+        Usage: /changelog [version] [change-type] [message]
+      '';
       commit = ''
         # Commit Command
 
         Create a git commit with proper message formatting.
-        Usage: /commit [message]'';
+        Usage: /commit [message]
+      '';
     };
   };
   nmt.script = ''