Bump version to 1.2.0 - #533

## Why

The release process isn't written down. The only documented step is the
docs deploy in `CONTRIBUTING.md`; the rest lives in people's heads and
in past bump PRs like #533.

## What changes

- New `RELEASING.md` with six steps: bump PR, check the Buildkite
artifacts, merge, publish the GitHub release with the tag, deploy the
docs, close the milestone.
- `CONTRIBUTING.md`'s "cutting a release" now links to it.

The steps come from how v1.2.0 and v1.2.0-beta.2 were actually cut:
#533's diff and checks, the release assets and flags, the `v1.2.0` tag
on the squash commit `7afbb1a`, and the `docs.yml` dispatches.

Deliberately **not** here: the `npm run i18n:download` step. That script
only exists once #544 lands, so a stacked follow-up adds it. The CI
check #543 suggests isn't in either PR.

## How to test this

Platforms: any. Nothing runs; this is a docs change. Read `RELEASING.md`
against the last release:

1. Open #533. Step 1's three version fields, check commands and "exactly
three changed lines" match its diff and test steps.
2. Run `gh release view v1.2.0 --json assets -q '.assets[].name'` from
anywhere. The three names match step 2's list.
3. Run `git rev-parse v1.2.0` from the repository root. It prints
`7afbb1a…`, #533's squash commit, which is what step 4 says to tag.
4. Click the `CONTRIBUTING.md#the-documentation-site` link in step 5 and
the `RELEASING.md` link in `CONTRIBUTING.md`. Both land in the right
place.

**What must not have happened:** no file other than `RELEASING.md` and
one line of `CONTRIBUTING.md` changes.

## Risks and limitations

- **Where the shipped artifacts come from needs confirming.** Buildkite
reports no commit status through the GitHub API on trunk or PR commits,
so I couldn't see which build v1.2.0's files came from. The doc says to
take them from the bump PR's head build, then merge only if that head is
current with trunk, so the tagged commit has the same content (true for
#533: its head tree equals `7afbb1a`'s). If Buildkite also builds trunk
and the files came from there, step 2 and 3 should say so.
- The `app.asar` and `CFBundleShortVersionString` checks are copied from
#533's post-merge checks; the doc moves them before the merge.
- Pre-PR review: 2 [fix here], both fixed.

## Related

Part of #543. The translation step follows in a PR stacked on #544.

---

<details>
<summary>Review outcome (required — see AGENTS.md)</summary>

2 [fix here] · 0 [follow-up]. Both fixed.

1. Step 3 said to re-check the artifacts after updating the branch but
not to download them again, so the files from the old head could ship.
The download now happens in step 3, after the last check.
2. Step 6 said to close the `vX.Y.Z` milestone for every release, but
most betas had none. It now says "if the release has a milestone", and
to leave the stable one open until the stable release.

Also from the review's notes: past release notes end with a
**Downloads** section. Step 4 says so now.

- **Review:** completed; separate-context Explore agent; reviewed
`760de7b` against base trunk `0dc54da`; outcome above.
- **Since review:** `760de7b → 50e5455`, the two fixes above. Not
re-reviewed. `npm run lint` clean; `npm test` 1676 tests (1674 pass, 2
skipped) on `760de7b`, and the later commit touches only Markdown.
- **CodeRabbit:** not yet requested.

</details>

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## Why

41 commits have reached trunk since `v1.2.1`, and milestone
`v2.0.0-beta.1` has 77 closed issues and none open. The main changes are
the redesign on the WordPress Design System (#591), a settings dialog
with language, theme, PHP version and debug flags (#638 to #651), and
translation of the whole interface (#634 to #664). This is a beta: it
ships with `--prerelease`, so `releases/latest` stays on `v1.2.1`.

`electron-builder` names the artifacts after `package.json`, so the
version has to move before the tag and artifacts exist.

## What changes

- **The version.** The three root version fields in `package.json` and
`package-lock.json` move from `1.2.1` to `2.0.0-beta.1`. No dependency
versions change.
- **The first shipped translations.** `npm run i18n:download` added a
catalog for each of the seven locales at 80% or more on
translate.wordpress.org. None was removed, and no right-to-left locale
was held back.

| Locale | Translated | Strings |
| --- | --- | --- |
| cs | 94% | 1001 |
| en-ca | 100% | 1062 |
| en-gb | 100% | 1062 |
| gl | 100% | 1062 |
| hu | 100% | 1062 |
| nl | 100% | 1062 |
| pl | 96% | 1028 |

## How to test this

**Platforms:** any for steps 1 to 3. The artifact checks need macOS for
`Info.plist` and Windows for the `.exe`.

**Starting state:** this branch checked out with `node_modules`
installed. Run every command from the repository root.

1. `npm run lint` is clean and `npm test` passes.
2. `node -e "console.log(require('./package.json').version)"` prints
`2.0.0-beta.1`.
3. `python3 -c "import
json;d=json.load(open('package-lock.json'));print(d['version'],
d['packages']['']['version'])"` prints `2.0.0-beta.1 2.0.0-beta.1`.
4. On the Buildkite build for the current head, the artifacts are named
`wordpress-contributor-toolkit-2.0.0-beta.1-*`. The `app.asar` root
holds exactly `node_modules`, `package.json` and `src`, with npm 11
under `node_modules/npm` and no `.codesigning`. On macOS,
`CFBundleShortVersionString` reads `2.0.0-beta.1`.
5. In the installed app, set the language to Dutch in the settings and
relaunch. The interface is in Dutch.

**What must not have happened:** no dependency version may change. `git
diff trunk -- . ':!src/languages'` shows exactly three changed lines,
all of them the root version.

## Risks and limitations

`npm test` run from an interactive terminal fails two integration tests
on macOS. That is a test bug, fixed separately in #680. It does not
affect CI or the app.

## Related

Milestone `v2.0.0-beta.1`. Release notes follow on the release.

---

<details>
<summary>Review outcome (required — see AGENTS.md)</summary>

0 [fix here] · 0 [follow-up]. Automatic checks only, as in #533: this
change has no logic to review.

- **Review:** completed, deterministic layer, in session; head `9853082`
/ base `5d5f701` (trunk). `npm run lint` passes, `npm test` passes (2098
tests, 0 failures, 2 skipped), both files are valid JSON, and the diff
outside `src/languages/` is the three root version fields. Outcome:
nothing to fix.
- **Since review:** none.

</details>

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