Enable maintenance-free README translation sync

[SkyWuZJU]

Never hand-maintain a translated README again

Translated READMEs are accurate exactly once: the day they land. They go stale
the next time you edit README.md, and keeping them current is
manual work forever.

The HY MT2 README Sync GitHub App takes that work
off your plate. It watches your source README and, whenever it changes, opens a
pull request that re-translates only the paragraphs that actually changed.
It only ever opens pull requests — it never commits to your default branch
and never merges anything itself, so nothing lands without your review.

It's free. Translation runs on our infrastructure using
Tencent Hunyuan MT (HY MT2), so there's no repository
secret, no Actions settings toggle, and no CI minutes on your side.

Turning it on

1. Merge this PR — it carries the config the App needs, already written for you.
2. Install the App and select this repository. install
3. Grant it contents and pull-request access — it only ever writes its own
   pull-request branch.

That's the whole setup.

What's in this PR

One data file — .github/hy-mt2.json — recording your source
README (README.md) and the translated READMEs to keep in sync:

• README_zh-CN.md (zh-CN)
• README_ja.md (ja)
• README_es.md (es)

It is pure data: no workflow, no script, no secret, and no code. Only the
App ever reads it, so until you install the App this PR changes nothing.

No lock-in

• Uninstall the App whenever you like and the syncing simply stops.
• Want different languages later? Edit the targets in
  .github/hy-mt2.json — that file is the only thing the App reads.
• Not interested at all? Closing this PR costs you nothing and leaves the
  translations from the companion PR untouched.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/vscode

Inputs:

• PR number: 2090
• Commit SHA: f844adeb82765d8b960c83a3c9f48935093704bf

Checks will be approved automatically on success.