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/js-cache-property-access.md

@@ -0,0 +1,28 @@
+---
+title: Cache Property Access in Loops
+impact: LOW-MEDIUM
+impactDescription: reduces lookups
+tags: javascript, loops, optimization, caching
+---
+
+## Cache Property Access in Loops
+
+Cache object property lookups in hot paths.
+
+**Incorrect (3 lookups × N iterations):**
+
+```typescript
+for (let i = 0; i < arr.length; i++) {
+  process(obj.config.settings.value)
+}
+```
+
+**Correct (1 lookup total):**
+
+```typescript
+const value = obj.config.settings.value
+const len = arr.length
+for (let i = 0; i < len; i++) {
+  process(value)
+}
+```