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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