feat(create-agent-skills): rewrite to match Anthropic official spec

The previous version incorrectly recommended XML tags for skill body content.
Anthropic's official specification uses standard markdown headings.

Changes:
- SKILL.md: Complete rewrite using markdown format, not XML tags
- Added references/official-spec.md from code.claude.com/docs/en/skills
- Added references/best-practices.md from platform.claude.com
- Removed obsolete use-xml-tags.md
- Updated naming to gerund form (creating-agent-skills)
- Descriptions now third person with "what" and "when"

BREAKING: If you followed the old XML tag guidance, convert to markdown headings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/compound-engineering/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "compound-engineering",
-  "version": "2.19.0",
+  "version": "2.20.0",
   "description": "AI-powered development tools. 27 agents, 20 commands, 13 skills, 2 MCP servers for code review, research, design, and workflow automation.",
   "author": {
     "name": "Kieran Klaassen",