docs(ci): the bump-level rule — an additive widening of a published surface is at least minor (#15380)

Record the maintainer's ruling (2026-09-04, decision batch #35) where authors
already read the changeset rules: the `Check Changeset` step's route-1 prose in
`.github/workflows/pr-automation.yml`, plus a one-line cross-reference from the
header of `scripts/check-changeset-no-major.mjs`, which already carries the
launch-window reasoning the rule's last sentence depends on.

Prose only: no new gate, no behaviour change, nothing retro-fixed.


Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Co-authored-by: Claude <noreply@anthropic.com>

Author: baozhoutao

.github/workflows/pr-automation.yml

@@ -664,6 +664,23 @@ jobs:
             1. It releases something
                -> run 'pnpm changeset' and name the packages it releases.
 
+               WHICH LEVEL: A purely additive widening of a published package's
+               public surface (a new exported symbol on an `index`, a new accepted
+               key or value) takes at least `minor`. The commit type may raise a
+               bump but never lower it below what the act requires; a `fix(` that
+               widens an index is therefore `minor`, and a `fix(` that changes no
+               public surface stays `patch`. During the launch window `major` stays
+               refused by `check-changeset-no-major` and breaking-ness is carried by
+               the BREAKING banner plus the ADR-0087 disposition, not by the level.
+               Ruled by the maintainer on 2026-09-04 (decision batch #35) on #15294,
+               which recorded two contract reviews reading the repo's own history to
+               OPPOSITE bumps for the same additive act. The commit type is not the
+               discriminator it looked like: it correlates with the act, and when
+               they disagree the act wins. The 64 historical `patch` precedents are
+               pre-rule and nothing is retro-fixed. This is prose, not a gate -- no
+               check computes it; `check-changeset-no-major.mjs` refuses `major` and
+               says why in its header, and the two remaining levels are yours.
+
             2. It releases nothing (.github/, .claude/, skills/, docs/, content/,
                examples/, tests-only, and the like)
                -> apply the 'skip-changeset' label.   <<< PREFERRED

scripts/check-changeset-no-major.mjs

@@ -51,6 +51,13 @@
  * change that states neither). They are not documentation niceties — during the
  * window they are the only signal there is.
  *
+ * CHOOSING BETWEEN THE TWO LEVELS THIS GUARD LEAVES: a purely additive widening
+ * of a published package's public surface takes at least `minor`, and the commit
+ * type may raise a bump but never lower it — the rule is written out in full in
+ * the `Check Changeset` step's prose in `.github/workflows/pr-automation.yml`
+ * ("WHICH LEVEL"), where the author who is told a changeset is missing reads it
+ * (maintainer ruling, 2026-09-04 decision batch #35, on #15294).
+ *
  * ⇒ **THIS GUARD IS WHAT GETS DISARMED AT GA.** The end condition lives in the
  * file that enforces the convention, rather than in a prose home elsewhere, so
  * that declared = enforced stays in one place: whoever closes the launch window