docs(changeset): correct one stale clause in the declaredCode changeset (#12502)

`.changeset/package-door-declared-code.md` closed with "the shared `sendError`'s
`extra` does not admit it yet". That clause was already false when it merged:
PR #12477 (#12404, `0e5fe7fc0`) widened the `extra` Pick to admit `userMessage`
and landed BEFORE `14b114553`. Both changesets are unreleased and compile into
the same CHANGELOG, so this fixes a document before it is published rather than
rewriting history — and it is fixed in place, because a correction sitting
beside the false sentence would ship both to the reader.

One clause. The rest of that changeset's prose is untouched, and no other
changeset is touched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UjujZN219uFzBhSYfMykCd

Author: claude

.changeset/package-door-declared-code.md

@@ -38,4 +38,4 @@ scoped to the prose by its own contract (`status`, `code` and `details` untouche
 either.
 
 ⛔ `userMessage` is deliberately NOT threaded here — the shared `sendError`'s `extra`
-does not admit it yet, and that channel is tracked separately.
+has admitted it since #12404, and that channel is threaded separately, in #12502.