docs: clarify Node and Corepack setup

[koriyoshi2041]

Summary

• clarify that contributors should use Node 22 LTS for Corepack-based setup
• document the Node 25+ Corepack absence and direct pnpm fallback
• add setup troubleshooting entries for missing pnpm, missing Corepack, Node version drift, and CLI path issues
• update the standalone pnpm installer example to the repo's pinned pnpm version

Verification

• git diff --check

Fixes #3270
Fixes #3288

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[imdreamrunner]

Thanks for tackling this - the troubleshooting section directly addresses the confusion in #3288.

One suggestion: I'd keep "Node 22+ (LTS)" rather than hard-pinning to exactly 22. pnpm just needs Node 22+, and a single pinned version ages out - Node 24 is already the current Active LTS, with 26 landing this fall. Recommending "an active LTS line" keeps this durable without bumping the docs each cycle.

I'd also frame the Node choice as independent from Corepack. Right now it reads a bit like "use 22 so Corepack works," but we shouldn't pin Node just to keep Corepack around. The TSC voted to stop shipping Corepack with Node 25+ - that's the intended direction, and it's fine. Corepack is a nice convenience (auto-honoring the pinned packageManager version), but not a requirement; the project installs fine with pnpm installed directly.

So I'd suggest: recommend an active LTS (22+), with two equally-valid ways to get pnpm -

1. Install pnpm directly (brew, standalone script, or npm i -g pnpm@10), or
2. Use Corepack for the auto-pinning - and if your Node no longer bundles it (25+), just install it manually first:

npm install -g corepack
corepack enable

That way the Corepack path still works for anyone who wants it, regardless of Node version, and we're not tying our recommended Node version to a tool that's being unbundled.

[koriyoshi2041]

Updated this to recommend an active LTS line (22+) instead of hard-pinning exactly 22, and split pnpm setup into two paths: direct pnpm install or Corepack for the pinned version. I also added the manual Corepack install path for Node versions that no longer bundle it.

git diff --check is clean. The remaining CLA check is still on me/account-side.