Build/Test Tools: Install the npm version required by package.json in GitHub Actions

[manzoorwanijk]

Adds a setup-npm action, mirroring WordPress/gutenberg#82775 and WordPress/gutenberg#82823, so CI installs the npm required by devEngines.packageManager even when Node.js bundles an older one. It also adds the same devEngines field as Gutenberg to package.json.

It will allow us to use the latest version of npm in CI without the EBADDEVENGINES error.

• setup-npm owns the npm cache, because actions/setup-node resolves it with the bundled npm, which fails with EBADDEVENGINES once a newer npm is required.
• Release branches run these reusable workflows from trunk against their own checkout, which lacks the action. A hashFiles gate skips it there.
• Release branches lose the npm download cache. They run rarely, and setup-node's exact key rarely hit anyway.
• Callers' paths filters now include .github/actions/setup-npm/**.

cc @desrosj: unrelated, but 6.9's test-build-processes.yml omits gutenberg-branch, so it builds Gutenberg trunk on Node.js 20 and every Gutenberg job fails with EBADENGINE. Passing gutenberg-branch: 'wp/6.9', as 6.5 to 6.8 do, we should fix it.

Trac ticket: https://core.trac.wordpress.org/ticket/66099

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Implementation and this description, reviewed by me.

🤖 Generated with Claude Code

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props manzoorwanijk, mciampini.

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[manzoorwanijk]

I can't request a review, so CCing you all

@desrosj @lancewillett @aduth @ciampo @jsnajdr @tyxla

[github-actions]

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[ciampo]

LGTM 🚀

Makes sense, as per instructions.

Not sure if we should wait for more approvals, especially for folks who continute to this repo more frequently.