fix(react-best-practices): warn about Set/Map recreation in function scope

[mvanhorn]

Summary

Adds a warning about the common pitfall of recreating Set/Map inside functions called repeatedly, which negates the O(1) lookup benefit.

Why this matters

The current rule shows converting arrays to Set for O(1) lookups but doesn't warn about a common pitfall: creating the Set inside a function body that runs on every call (#54). This means Set construction (O(n)) runs every time, negating the performance benefit. An AI agent or junior developer following the rule literally could produce this anti-pattern.

Changes

• Added a "Common Pitfall" section showing the anti-pattern of Set recreation inside functions
• Added the correct pattern of hoisting Set to module level
• Kept existing rule content unchanged (additive change only)

Testing

Reviewed rule formatting for consistency with other rule files in the repo.

Fixes #54

This contribution was developed with AI assistance (Claude Code).

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[HonzysClawdbot]

This PR adds a warning about Set/Map recreation in function scope - a common pitfall that negates the O(1) lookup benefit. ✅ CI is passing (Vercel Agent Review).

Fixes #54. Important safety addition - can be merged safely.

[mvanhorn]

Hey, checking in on my open agent-skills PRs:

• fix(react-best-practices): warn about Set/Map recreation in function scope #180 - Set/Map recreation warning (22 lines)
• fix(react-native-skills): add platform notes for iOS-only ScrollView props #181 - iOS-only ScrollView platform notes (61 lines)
• feat(react-best-practices): add Server Action & Form pattern rules #192 - Server Action & Form pattern rules (408 lines)

All have passing CI. Let me know if any need changes.