Add marketplace metadata for plugin registry

[TheMostlyGreat]

Summary

Add .claude-plugin/marketplace.json for public plugin registry listing.

Background

Claude Code has a plugin marketplace. Adding marketplace metadata enables:

• Public discoverability
• Version tracking
• Installation statistics
• Category/tag organization

Implementation

1. Create .claude-plugin/marketplace.json:

   {
     "category": "development-tools",
     "tags": ["tdd", "quality", "architecture", "workflows"],
     "screenshots": [],
     "featured": false,
     "pricing": "free",
     "supportUrl": "https://github.com/yourorg/safeword/issues",
     "changelog": "https://github.com/yourorg/safeword/blob/main/CHANGELOG.md"
   }

2. Add marketplace submission documentation
3. Consider what makes safeword discoverable (keywords, description)

Acceptance Criteria

• Valid marketplace.json created
• Metadata follows marketplace requirements
• Documentation for marketplace submission
• Plugin passes marketplace validation

Priority

Tier 3 - Low effort, nice for distribution.

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