feat(react-best-practices): add Server Action & Form pattern rules

[mvanhorn]

Summary

Adds 5 new rules to the Server-Side Performance and Re-render Optimization categories covering Server Action and Form patterns.

New rules

• server-useoptimistic - Use useOptimistic for instant UI feedback on mutations (HIGH)
• server-action-revalidation - Granular cache revalidation after mutations (HIGH)
• server-action-error-handling - Return error state instead of throwing (MEDIUM-HIGH)
• server-progressive-enhancement - Forms that work without JavaScript (MEDIUM)
• rerender-useformstatus - Pending state via useFormStatus without prop drilling (MEDIUM)

Why these rules

The skill has 8 server- rules but only server-auth-actions.md touches Server Actions directly (covering authentication). Server Actions are the primary mutation mechanism in Next.js App Router, and the patterns above are the most common sources of bugs and poor UX:

• Waiting for server response before updating UI (useOptimistic fixes this)
• Using revalidatePath('/') instead of granular revalidateTag after mutations
• Throwing errors in actions instead of returning error state (kills the form UI)
• Using onClick handlers instead of form actions (breaks without JS)
• Prop drilling pending state instead of using useFormStatus

Each rule follows the existing _template.md format with incorrect/correct TypeScript examples and references to React/Next.js docs.

Complements PR #62 (useActionState) - these rules cover different aspects of the Server Action lifecycle.

Changes

• 5 new rule files in skills/react-best-practices/rules/
• Updated SKILL.md Quick Reference to list new rules (64 -> 69 rules)

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

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