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/pr-reviewer/references/troubleshooting.md

@@ -0,0 +1,55 @@
+# Troubleshooting Guide
+
+Common issues and solutions for the PR Reviewer skill.
+
+## gh CLI Not Found
+
+Install GitHub CLI: https://cli.github.com/
+
+```bash
+# macOS
+brew install gh
+
+# Linux
+sudo apt install gh  # or yum, dnf, etc.
+
+# Authenticate
+gh auth login
+```
+
+## Permission Denied Errors
+
+Check authentication:
+
+```bash
+gh auth status
+gh auth refresh -s repo
+```
+
+## Invalid PR URL
+
+Ensure URL format: `https://github.com/owner/repo/pull/NUMBER`
+
+## Line Number Mismatch in Diff
+
+Inline comment line numbers are **relative to the diff**, not absolute file positions.
+Use `gh pr diff <number>` to see diff line numbers.
+
+## Rate Limit Errors
+
+```bash
+# Check rate limit
+gh api /rate_limit
+
+# Authenticated users get higher limits
+gh auth login
+```
+
+## Common Error Patterns
+
+| Error | Cause | Solution |
+|-------|-------|----------|
+| 401 Unauthorized | Token expired | Run `gh auth refresh` |
+| 403 Forbidden | Missing scope | Run `gh auth refresh -s repo` |
+| 404 Not Found | Private repo access | Verify repo permissions |
+| 422 Unprocessable | Invalid request | Check command arguments |