release: v2.1.2 — landing page TSX output, brand voice integration, docs update

- Landing page generator defaults to Next.js TSX + Tailwind CSS (4 design styles)
- Brand voice analyzer integrated into landing page generation workflow
- CHANGELOG, CLAUDE.md, README.md updated for v2.1.2
- All 13 plugin.json + marketplace.json bumped to 2.1.2
- Gemini/Codex skill indexes re-synced
- Backward compatible: --format html preserved, no breaking changes

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

engineering/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "engineering-advanced-skills",
   "description": "25 advanced engineering skills: agent designer, RAG architect, database designer, migration architect, observability designer, dependency auditor, release manager, API reviewer, CI/CD pipeline builder, MCP server builder, skill security auditor, performance profiler, and more",
-  "version": "2.1.1",
+  "version": "2.1.2",
   "author": {
     "name": "Alireza Rezvani",
     "url": "https://alirezarezvani.com"