feat: Initial agent-skills repo — 4 adapted skills for Mosaic Stack

Skills included:
- pr-reviewer: Adapted for Gitea/GitHub via platform-aware scripts
  (dropped fetch_pr_data.py and add_inline_comment.py, kept generate_review_files.py)
- code-review-excellence: Methodology and checklists (React, TS, Python, etc.)
- vercel-react-best-practices: 57 rules for React/Next.js performance
- tailwind-design-system: Tailwind CSS v4 patterns, CVA, design tokens

New shell scripts added to ~/.claude/scripts/git/:
- pr-diff.sh: Get PR diff (GitHub gh / Gitea API)
- pr-metadata.sh: Get PR metadata as normalized JSON

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/vercel-react-best-practices/rules/bundle-dynamic-imports.md

@@ -0,0 +1,35 @@
+---
+title: Dynamic Imports for Heavy Components
+impact: CRITICAL
+impactDescription: directly affects TTI and LCP
+tags: bundle, dynamic-import, code-splitting, next-dynamic
+---
+
+## Dynamic Imports for Heavy Components
+
+Use `next/dynamic` to lazy-load large components not needed on initial render.
+
+**Incorrect (Monaco bundles with main chunk ~300KB):**
+
+```tsx
+import { MonacoEditor } from './monaco-editor'
+
+function CodePanel({ code }: { code: string }) {
+  return <MonacoEditor value={code} />
+}
+```
+
+**Correct (Monaco loads on demand):**
+
+```tsx
+import dynamic from 'next/dynamic'
+
+const MonacoEditor = dynamic(
+  () => import('./monaco-editor').then(m => m.MonacoEditor),
+  { ssr: false }
+)
+
+function CodePanel({ code }: { code: string }) {
+  return <MonacoEditor value={code} />
+}
+```