docs: test types as users use them, and fix a bug everywhere it lives

Bugs that reached users or outside reviewers came from types used in
ways the tests did not try, such as a generic function of the user's or
a recipe exported from another module, and from fixes that left the same
pattern in another package or in the docs. AGENTS.md now lists the ways
each public type is checked, and asks a fix to search every package and
the docs for the pattern it fixes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: timphandev

AGENTS.md

@@ -229,6 +229,31 @@ public type has a type test (`expectTypeOf`, or `@ts-expect-error` for
 input that must be rejected). A test that only exercises the types still
 asserts the runtime outcome.
 
+**Test types the way users use them.** A type that works on one inline
+call can still fail in a user's project. Check each public type, in a
+test or in an app of `consumers`, in each way users reach it:
+
+- inferred from an inline argument, and from a value declared before the
+  call;
+- passed through a function of the user's that is generic over it, and
+  returned from that function;
+- exported from one module and used in another, with declarations
+  emitted, as an app that installs the packed package compiles it;
+- with `isolatedDeclarations`;
+- nested or composed several levels deep, where its declarations must
+  grow linearly;
+- with the oldest and the newest TypeScript that the packages support.
+
+When a feature adds a way to use a type, add that way to every type it
+applies to, and to this list.
+
+**Fix a bug everywhere it lives.** The packages and the docs share
+patterns: the same type helper, the same check, the same example. When
+you fix a bug, find the pattern that caused it, then search every
+package, the docs, the skills, and the READMEs for that pattern. Fix
+each place in the same change, with a test for each, or say why a place
+is not affected. List in the commit body where you looked.
+
 **Performance comes first.** A recipe and a slot recipe of any kind, a
 class name recipe and slot recipe, and a style recipe and slot style
 recipe must be faster with the cache than without it. The benchmarks