feat(core): add skills for react key rendering pattern

[zztnrudzz13]

Overview

• Add U18, a new React hook usage pattern covering stable keys for list and subtree identity.
• Extend the React hook review skill to consult a dedicated key rendering reference when reviewing lists, JSX arrays, state preservation/reset, remounting, or key-based rendering efficiency.
• Update English and Korean principles docs plus plugin README counts from U1-U17 to U1-U18.

Checklist

• Did you write the test code?
• Have you run yarn run fix to format and lint the code and docs?
• Have you run yarn run test:coverage to make sure there is no uncovered line?
• Did you write the JSDoc?

[changeset-bot]

🦋 Changeset detected

Latest commit: 777c0cb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
react-simplikit	Patch

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[github-actions]

Size Change: 0 B 🆕

Total Size: 0 B

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[kimyouknow]

Hi @zztnrudzz13 — thanks for this! The U18 content is genuinely high quality, and I appreciate that you kept the counts consistent across every file (16→17 patterns, 30→31 principles). Two small things before merge:

1. The changeset
.changeset/tough-peas-relate.md bumps react-simplikit: patch, but this PR only touches root docs/ and packages/plugin/ — neither ships in the npm package (packages/plugin has no package.json). With the changeset in place, merging would cut an empty react-simplikit release with no actual library change. Could you drop the changeset?

2. Title scope (totally minor)
The title is feat(core) but there's no packages/core change — maybe retitle to something like docs(plugin): ... so it doesn't read as a published core feature?

Thanks again for keeping the principles docs so polished!