fix(scripts): refuse a PR that modifies or deletes a changeset it did not add (#17712) (#18146)

Closes #17712

`Clause-②: no` — this card only makes a gate refuse MORE. It loosens no
accept set and widens no published surface.

Implements **item 1 (A′)** of the ruling on #17712 (director seat,
decision batch #130 item 3, 2026-09-13; maintainer's verbatim reply
「同意」). Item 2 (the `ISSUE-SLUG.md` naming line in governed dev-round
guidance) is #17748's and is not touched here. Item 3 — legacy names
untouched, diff shape only — is honoured: nothing in this diff reads a
changeset filename.

## The rule

A PR may not MODIFY or DELETE a `.changeset/*.md` that exists on the
merge base and was not added by this PR. Refused by name, one remedy:
**rename yours; restore theirs from base**.

"Added by this PR" is the file's absence on the merge base — which is
exactly what git's status letters already say, so the rule needs no
second reading and no filename predicate at all:

| row | meaning | verdict |
|:--|:--|:--|
| `A` | absent at the merge base | this PR's own file, always ok |
| `M` | present at the merge base, changed here | foreign, REFUSED |
| `D` | present at the merge base, gone here | foreign, REFUSED |

## Where it is wired, and why

Into `scripts/check-empty-changeset.mjs`, as a second axis answered in
the same run — the shape `check-changeset-no-major.mjs` already uses for
its two axes. **The PR touches no workflow file at all**, so it can be
armed normally.

Three things that come for free from that placement, each of which a
standalone script would have had to re-derive:

1. **The `changeset-release/main` exemption.** `changeset version`
DELETES every pending changeset, so the Version Packages PR is all-`D`
rows. Outside `changeset-check`'s job-level `if:`, this gate would be
structurally unsatisfiable on every release PR — the #4422 / #4894
shape, pointed at the release train.
2. **The merge base.** The `changeset-check` job already derives
`MERGE_BASE` and hands it to this script. That matters more for this
rule than for the others: run two-dot against a moving base tip instead
of the merge base and every changeset `main` gained while the PR sat
open reports as a `D` on this branch — an author refused by name for
deleting files they never touched. The self-test pins that false red as
a firing control beside the real reading.
3. **A self-test that cannot be skipped.** `check-empty-changeset.mjs
--self-test` runs unconditionally in `lint.yml` (#6509), outside the
`skip-changeset` exemption — which is where this gate's own fixtures
need to be, since a PR editing a CI-internal script is the textbook
`skip-changeset` case.

The file name still names rule 1 only. Renaming it would mean editing
the workflow steps that spawn it; the header says so rather than leaving
it to be noticed.

**Rename detection is OFF for this pass** (`--no-renames`), the opposite
of the `AMR` choice rule 1 makes in #7045 — and for the same underlying
reason, one letter over. Renaming somebody else's changeset DELETES
their release note at its path; with detection on, that deletion folds
into an `R` row and disappears. With it off the same edit reports `D
theirs` + `A yours` and the `D` is refused. It costs nothing in the
other direction: a PR renaming its OWN changeset across commits still
shows one `A` row, because the old path was never on the merge base
either.

## The four ruled cases, end to end on this repository

Driven on a throwaway worktree at this branch's tip, against real
commits (the gate reads commits, never the working tree):

| case | diff rows vs merge base | exit |
|:--|:--|--:|
| foreign `M` — overwrite a sibling's changeset | `M
.changeset/12271-published-entry-no-auto-transpile.md` | **1** |
| foreign `D` — delete theirs, add mine | `D .changeset/12271-...md` +
`A .changeset/17712-demo-only.md` | **1** |
| own `M` across commits — add in commit 1, reword in commit 2 | `A
.changeset/17712-demo-only.md` | **0** |
| new `A` | `A .changeset/17712-demo-only.md` | **0** |

The refusal, verbatim:

```
This PR changes a changeset it did not add:

   .changeset/12271-published-entry-no-auto-transpile.md
     present on the merge base and DELETED by this PR -- this is somebody else's release note

Remedy: rename yours; restore theirs from base.
```

plus a `::error file=...` annotation carrying the same remedy, so it
lands on the diff rather than only in a log.

## Ablation — the currently-false sentence this makes true

Same two fixtures, with `scripts/check-empty-changeset.mjs` restored to
its `origin/main` blob (`1c5638ace2`, marker count for `scanForeign` 0
on disk; restored to `24739e0fad`, marker count 12, `git diff HEAD`
empty):

| fixture | pre-change gate | post-change gate |
|:--|--:|--:|
| overwrite a sibling's changeset | **exit 0** | exit 1 |
| delete theirs + add mine | **exit 0** | exit 1 |

## Reverse-read, both directions

**Made false.** "`check-empty-changeset.mjs` exits 0 on any diff whose
`.changeset/*.md` rows are all non-empty at head." It no longer does —
case `M` above shows rule 1 printing its own green tick on the same run
that exits 1.

**Made true.** "A PR that changes or deletes another PR's release note
is refused by name, with a remedy." Previously false in every member of
the family: the only filename predicate in the changeset gates is prefix
+ extension, and `scan()`'s `--diff-filter=AMR` has no `D` at all, so a
deletion was invisible to all three. Pinned as an assertion, not just
asserted here: the foreign-`D` battery case also runs `scan()` on the
same fixture and requires **zero** rule-1 violations — the control that
this rule is not redundant with the one beside it.

## One premise in the card is not exactly right, measured

The card says the collision would pass "with **every gate green** on
both sides". Measured on real fixtures against `pr-automation.yml`'s own
`--diff-filter=A` count:

| shape | `ADDED` | pre-change verdict |
|:--|--:|:--|
| overwrite theirs, add nothing of your own | **0** | already RED today
— but as *"this PR doesn't add a changeset"* |
| overwrite theirs AND add your own | 1 | green |
| delete theirs, add your own | 1 | green |

So the narrowest shape of the incident was already refused — by the
wrong gate, naming the wrong problem, and with a remedy ("add a
changeset") that leaves the overwrite in place. The two shapes that
actually reach `main` were green. The card's conclusion stands; its
"every gate" is one row too strong, and the correction is recorded here
rather than left for the next reader of that thread.

## Self-test

New battery `A' (#17712): a changeset the PR did not add is neither
modified nor deleted` — **29 cases**, pinned in `SELF_TEST_BATTERIES`
and reached (the roster's own size floor moves 21 → 22). The
battery-floor machinery (#13489) is what makes "every case held"
distinguishable from "the cases never ran".

Beyond the four ruled cases it covers: renaming a foreign changeset
(with the rename-detection-ON control proving the `R` row really does
swallow the `D`); renaming your own changeset across commits;
`.changeset/README.md`; a nonsense control (a diff touching no
changeset); #6129 in this rule's direction, firing control first; #4690
(no merge base throws, never falls back); and that the rendered report
names the file and carries the remedy verbatim in both the body and the
annotation.

Every green in the battery carries a control that the fixture did what
it claims — a `0` from a diff that was empty for an unrelated reason is
a reading taken against nothing, not a pass.

```
✓ check-empty-changeset --self-test: 147 assertions over real temp git repos (real scan() path)
```

## Verification

At `eeb4012a17`:

- **`node scripts/pm/dispatch-gates.mjs --ran` — 34 derived families, 34
run, 0 NOT-MEASURED (derived, every entry carrying its exit code), 0
UNRUN.** All 34 exit 0. Re-derived after `git fetch origin main`; the
set did not move.
- **`npx eslint . --no-inline-config --format json` — exit 0 over 6751
files, 0 errors, 0 warnings.** The whole-repo run, not a narrowing: the
population is eslint's own count from its `--format json` output, and
this file is in it.
- Control-character self scan over the edited file: 0 matches (firing
control on a vertical tab: 1 match).
- `node scripts/check-empty-changeset.mjs --base origin/main` on this
branch: both axes green.

No package is touched, so there is no dependency closure to build and no
package test suite affected; `scripts/` is not TypeScript and no
`*.test.ts` in the tree names this script (control: `check-nul-bytes`
names three).

## No changeset — `skip-changeset`

Measured rather than asserted: **0** published (non-`private`) packages
have a `files[]` entry that would ship anything under `scripts/`. Firing
control: **70** published packages ship `dist/`. Nothing published
moves, so the label is the correct route and an empty-frontmatter
changeset would be refused by rule 1 of this very script.

## 验收备注

Two cells are recorded rather than implied, both inherited from the job
this step lives in and neither newly opened by it:

- A PR carrying `skip-changeset` is exempt from the whole
`changeset-check` job, so it is exempt from this rule too. Identical in
shape to the cell `check-empty-changeset.mjs` already records for rule
1, and with the same reasoning: a consistent exemption beats a gate that
reds one PR and greens an identical one. Motive is thin — a PR with the
label adds no changeset of its own, so it has nothing to collide with —
but deletion is still reachable under it.
- `pr-automation.yml` runs on `pull_request` only, so this gate does not
re-run in the merge queue. There is a recorded ⛔ in `lint.yml`'s `on:`
block against adding `merge_group` to advisory workflows, so closing
that would be a decision, not a wiring fix. What the ruling's
"re-evaluated against the current base" needs is satisfied on the PR
leg: the base is recomputed on every `synchronize`, and it is a merge
base, never a branch tip.

One maintainer-only question, deliberately left un-decided here: the
ruling names no escape hatch for **deliberately** correcting somebody
else's release note on `main`. The report tells such an author to say so
on the PR and get it confirmed. If a mechanical route is wanted instead,
that is a decision about release integrity and belongs to you, not to
this PR.

Noted, not filed: `scan()` and `scanForeign()` now hold two
near-identical merge-base preambles, including the same throw text.
Whoever next touches a third axis in this family will want one helper —
no PR or person is queued on this file today, so there is no carrier:
承接者:无.

---
_Generated by [Claude
Code](https://claude.ai/code/session_012GKcPZbMoGq7WPzKLfRBTU)_

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

Author: baozhoutao