Default implementation for system commands.

Opens files in the system's default browser. This is used during interactive diff resolution to show .good.html and .bad.html files side by side.

Mocking for CI

In CI environments, you typically want to mock this module to avoid opening browsers:

# test/test_helper.exs
Mox.defmock(Excessibility.SystemMock, for: Excessibility.SystemBehaviour)
Application.put_env(:excessibility, :system_mod, Excessibility.SystemMock)

# In tests
stub(Excessibility.SystemMock, :open_with_system_cmd, fn _path -> :ok end)