docs(docs-audit): record why CHANGELOG.md is NOT added to the drift exclusion

#4091 dropped test files before deriving the changed-package roots, because a
test cannot make an implementation-accuracy doc stale. `packages/*/CHANGELOG.md`
is the obvious next candidate by the same argument — release notes define
behaviour no more than a test does — and extending the exclusion there is a
provable no-op. Measured before writing anything down:

- `chore: version packages` is the only PR class that mass-touches those files,
  and it runs no GitHub Actions at all. `changesets/action` opens it with the
  repo's GITHUB_TOKEN, and GitHub does not create workflow runs from
  GITHUB_TOKEN-authored events. #3910 carries exactly one check run, from
  Vercel's own app — `ci.yml` matches that PR unconditionally and still never
  fired. So the drift gate never sees a release PR to be noisy on.
- Even if it did, `changeset version` writes `package.json` beside every
  `CHANGELOG.md` it appends to (45 against 46 on page 1 of #3910's diff), so
  dropping the CHANGELOGs would leave the derived package-root set identical.

The bump is not unverified, just verified later: `ci.yml` and `lint.yml` both run
on `push: main`, and `release.yml` gates publish on a green build — worth stating
so the absent PR checks are not read as a hole.

Recorded in the README beside the exclusion it neighbours, so the next reader
does not spend a PR rediscovering it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QoA8AV99Ss1RRkLLAYmDzq

Author: claude

.changeset/docs-audit-changelog-non-exclusion.md

@@ -0,0 +1,18 @@
+---
+---
+
+Docs only — no package changes, nothing to release.
+
+Records why `packages/*/CHANGELOG.md` is **not** added to the docs-drift mapper's test-file
+exclusion (#4091). It reads as the obvious next narrowing and is a provable no-op:
+
+- `chore: version packages` is the only PR class that mass-touches those files, and it runs
+  no GitHub Actions — `changesets/action` opens it with the repo `GITHUB_TOKEN`, which by
+  design triggers no workflow runs. Measured on #3910: one check run, Vercel's own app.
+  The version bump is still verified, just post-merge (`ci.yml` / `lint.yml` on `push: main`,
+  and `release.yml` gates publish on a green build).
+- `changeset version` writes `package.json` beside every `CHANGELOG.md` it appends to
+  (45 vs 46 on page 1 of #3910's diff), so excluding the CHANGELOGs would leave the derived
+  package-root set bit-identical anyway.
+
+Written down so the next reader does not spend a PR rediscovering it as a gap.

scripts/docs-audit/README.md

@@ -41,6 +41,24 @@ the PR where it is right. The count of excluded files is reported in the summary
 pins the matcher against paths that must and must not match (`commands/test.ts` is
 implementation; `foo.conformance.test.ts` is not).
 
+**And one deliberate non-exclusion:** `packages/*/CHANGELOG.md` stays counted, even though
+release notes define behaviour no more than a test does. Extending the exclusion there
+looks like the obvious next step and is a provable no-op, for two independent reasons:
+
+1. The only PR class that mass-touches those files is `chore: version packages`, and it
+   runs **no GitHub Actions at all** — `changesets/action` opens it with the repo's
+   `GITHUB_TOKEN`, and GitHub does not trigger workflow runs from `GITHUB_TOKEN`-authored
+   events. Measured on #3910: one check run, from Vercel's own app. So this gate never
+   sees a release PR to be noisy on. (The bump is still verified — `ci.yml` and `lint.yml`
+   both run on `push: main`, and `release.yml` gates publish on a green build.)
+2. Even if it did run, `changeset version` writes `package.json` next to every
+   `CHANGELOG.md` it appends to — 45 of the former against 46 of the latter on the first
+   page of #3910's diff — so dropping the CHANGELOGs would leave the derived package-root
+   set bit-identical.
+
+A hand-edited CHANGELOG outside a release is also close to nonexistent in practice. Left
+counted, and recorded here so the idea is not rediscovered as a gap.
+
 ## 2. CI gate — `.github/workflows/docs-drift-check.yml`
 
 On any PR that touches `packages/**`, runs `affected-docs.mjs` against the base branch