[WIP] Update documentation for dual-catalog system of extensions

[Copilot]

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Document dual-catalog system for extensions</issue_title>
<issue_description>## Summary

This issue tracks the documentation updates to clarify the dual-catalog system for Spec Kit extensions.

Background

Spec Kit uses two catalog files:

• catalog.json - Organization's curated catalog (empty by default)
• catalog.community.json - Reference catalog of community extensions

The current documentation doesn't clearly explain this distinction, which can confuse users about how to discover and install extensions.

Changes Needed

Update extension documentation to:

• Clarify the purpose and distinction of both catalogs
• Document the discovery → review → curate → install workflow
• Update EXTENSION-DEVELOPMENT-GUIDE.md
• Update EXTENSION-PUBLISHING-GUIDE.md
• Update EXTENSION-USER-GUIDE.md
• Update extensions/README.md
• Update RFC-EXTENSION-SYSTEM.md
• Document SPECKIT_CATALOG_URL environment variable
• Fix outdated GitHub organization references (statsperform → github)

Files Modified

1. extensions/EXTENSION-DEVELOPMENT-GUIDE.md - Updated Option 3 to explain community catalog
2. extensions/EXTENSION-PUBLISHING-GUIDE.md - Added dual-catalog workflow
3. extensions/EXTENSION-USER-GUIDE.md - Added catalog selection guidance
4. extensions/README.md - Comprehensive catalog explanation
5. extensions/RFC-EXTENSION-SYSTEM.md - Dual-catalog design and implementation

Impact

• Clearer understanding of extension discovery and installation
• Better guidance for extension publishers
• Improved documentation consistency
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Document dual-catalog system for extensions #1687

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