Document TuneToken behavior for custom checks

Author: mb706

AGENTS.md

@@ -90,6 +90,7 @@ Straightforwardness: Avoid ideological adherence to other programming principles
 
 - R unit tests in this repo assume helper `expect_man_exists()` is available. If you need to call it in a new test and you are working without mlr3pipelines installed, define a local fallback at the top of that test file before `expect_learner()` is used.
 - Revdep helper scripts live in `attic/revdeps/`. `download_revdeps.R` downloads reverse dependency source tarballs; `install_revdep_suggests.R` installs Suggests for those revdeps without pulling the revdeps themselves.
+- When writing `paradox::ParamSet` custom checks (e.g. `p_uty(custom_check = ...)`), you do not need to special-case `TuneToken`s. `paradox` skips custom validators for `TuneToken` inputs before evaluating them, so the check only sees concrete values.
 
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