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/server-serialization.md

@@ -0,0 +1,38 @@
+---
+title: Minimize Serialization at RSC Boundaries
+impact: HIGH
+impactDescription: reduces data transfer size
+tags: server, rsc, serialization, props
+---
+
+## Minimize Serialization at RSC Boundaries
+
+The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses.
+
+**Incorrect (serializes all 50 fields):**
+
+```tsx
+async function Page() {
+  const user = await fetchUser()  // 50 fields
+  return <Profile user={user} />
+}
+
+'use client'
+function Profile({ user }: { user: User }) {
+  return <div>{user.name}</div>  // uses 1 field
+}
+```
+
+**Correct (serializes only 1 field):**
+
+```tsx
+async function Page() {
+  const user = await fetchUser()
+  return <Profile name={user.name} />
+}
+
+'use client'
+function Profile({ name }: { name: string }) {
+  return <div>{name}</div>
+}
+```