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.
Include various tests for the agents and command file generation and
different ways of configuring it.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>