ci(nrg): generate README.md and README.zh.md from a single template

[andriishin]

What this changes

README.md and README.zh.md are currently maintained as two separate hand-edited files. Every commit that touches one needs a parallel edit in the other (and the git history confirms: every README commit so far has been a co-edit by you). This PR consolidates them into a single source — README.src.md — and adds a GitHub Action (nanolaba/nrg-action@v1) that regenerates both whenever the template changes.

After the merge: both rendered files on GitHub are unchanged; future content edits land in README.src.md once, and the bot updates both files in a single follow-up commit.

Byte-perfect preservation

Locally regenerated with NRG 1.2 — diff vs current master:

README.md       3 metadata-header insertions (only)
README.zh.md    3 metadata-header insertions (only)

No translation content, link, code block, locale-specific value (TZ, ATK_LOCALE, ATK_SITE_DEFAULT), heading, or image changed.

The 3-line metadata header marks each file as auto-generated:

<!-- This file was automatically generated by Nanolaba Readme Generator (NRG) 1.2 -->
<!-- Visit https://github.com/nanolaba/readme-generator for details -->

It signals to future contributors not to hand-edit, and the drift-check job below enforces it on PRs.

How the template works

• Untagged lines render in all languages — used for shared bits like badges, the <p align=\"center\">/<img> block, prettier-ignore markers, the contrib.rocks/repobeats/starchart images, and blank lines.
• Lines tagged <!--en--> or <!--zh--> only render in that language. Used for translated text, locale-specific code-block values (TZ=America/New_York vs Asia/Shanghai, ATK_LOCALE=en vs zh-CN, ATK_SITE_DEFAULT='Artalk Blog' vs 'Artalk 的博客'), and the Learn More → URL pointer (/en/ vs /zh/).
• File-name mapping: default lang → README.md, configured nrg.fileNamePattern.zh=README.zh.md.

Editing future README updates becomes: open README.src.md, edit the <!--en--> line, edit the <!--zh--> line, push. Drift-check on the PR catches if you forget the second edit (it just refuses to pass until both files reflect the template).

Adding a third language later (Japanese, etc.) is purely additive — append ja to <!--@nrg.languages=...-->, add <!--@nrg.fileNamePattern.ja=README.ja.md-->, and add <!--ja-->-tagged lines next to existing <!--en--> lines. Existing files stay untouched.

CI behavior

Two jobs in .github/workflows/nrg.yml:

• regenerate (push to master, when README.src.md or the workflow itself changes) — regenerates both READMEs and commits them as github-actions[bot].
• drift-check (pull_request, when any README*.md or the template changes) — fails the PR if a contributor hand-edited a generated file. The failure log shows a clear diff: 'regenerated content differs from on-disk; edit README.src.md instead.'

Both jobs pinned to nrg-version: '1.2'. No conflict with existing CI — test-docs.yml runs prettier/eslint on docs/** only, not on root README files.

If you don't want this

Easy to revert — drop README.src.md, drop .github/workflows/nrg.yml, the existing README.md and README.zh.md are byte-identical except for the auto-generated header (which is also trivial to strip). Happy to discuss alternative shapes if the byte-perfect-mirror approach isn't right for your workflow.