Convert requirements-clarity to plugin format with English prompts

Changes:
- Migrate from .claude/skills/ to .claude/plugins/ structure
- Add claude.json plugin metadata
- Create instructions.md with all English prompts (no Chinese)
- Update README.md for plugin distribution
- Update .gitignore to allow .claude/plugins/

Plugin structure:
- claude.json: Plugin metadata (name, version, components)
- instructions.md: Main skill prompt (100% English)
- README.md: Plugin documentation and usage guide

Maintains all functionality:
- 100-point scoring system
- Iterative clarification (≥90 threshold)
- PRD generation with 4 sections
- Auto-activation on vague requirements

Fixes #17

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

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

.gitignore

@@ -1,4 +1,4 @@
 CLAUDE.md
 
 .claude/*
-!.claude/skills/
+!.claude/plugins/