docs: point to .node-version instead of restating it

[altaywtf]

Problem

CONTRIBUTING restated the Node range (>=24.19.0) that .node-version and engines.node already declare.

Solution

Point contributors at .node-version, and link the pnpm install page.

Proof

pnpm run check passes; the pre-push verify gate passed.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-23T16:02:42.114567Z	df48374	PR opened

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[Copilot]

Copilot review overview

🟢 Approval recommended

The change is a straightforward documentation update with valid links and no behavioral impact on the codebase.

Review effort: Lite
Findings: None

What changed in this PR

This PR updates contributor onboarding docs to avoid duplicating the project’s Node.js version requirement by pointing readers to the single source of truth in .node-version, and adds a direct link to pnpm installation instructions.

Changes:

• Replaces the hardcoded Node version text in CONTRIBUTING.md with a link to .node-version.
• Links to the official pnpm installation page to help contributors install pnpm correctly.

File	Description
CONTRIBUTING.md	Removes duplicated Node version requirement and replaces it with links to .node-version and pnpm installation docs.

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