docs(agent): add React performance guidance

[beruro]

Summary

• vendor an ORGII-specific React 19 performance guidance overlay pinned to Vercel Engineering upstream
• classify Next.js, RSC, SSR, SWR, and server-only recommendations as adapted or inapplicable
• route performance-focused React work through the new skill without making it a gate for routine UI edits
• preserve the existing org2-performance-guard guidance and allow both workspace skills through .gitignore

Verification

• pre-commit formatting passed for Markdown files
• existing .orgii/skills/org2-performance-guard/** files are unchanged
• both workspace skill directories remain trackable
• git diff --check passed