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.
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