Built-in skill — ExUnit, Mox and property-based testing.
Injects a system-prompt section with examples for mirroring lib/ in test/
with async: true, setup blocks that return maps for pattern matching,
mocking only external boundaries through Mox behaviours plus
verify_on_exit!, StreamData properties, @tag :integration, and test names
that describe behaviour rather than implementation.
Activates on prompts containing "exunit", "elixir test", "mix test", "mox",
"test elixir", "property test" or "stream_data". Group :testing;
tags :elixir, :testing, :exunit, :mox.