chore: add Claude Code skills and unignore .claude/skills

[dschom]

Because

• We want to start adding some claude skills

This pull request

• unignores .claude/skills/ in .gitignore so skills are tracked in version control.
• security-review: OWASP, secrets, XSS/injection, CORS, crypto, OTP/TOTP, session replay, rate limiting, PII, Docker, CI
• check-docs: grammar, typos, JSDoc, README, API docs, changelog summary
• check-smells: design, implementation, test, and dependency smells
• check-react: component design, hooks, performance, a11y, state management
• check-githistory: cross-references current changes against git history for regressions
• explain-code: architecture, annotated walkthrough, gotchas for experienced engineers

Issue that this pull request solves

Closes: (issue number)

Checklist

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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Other information (Optional)

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