docs: Document dual-catalog system for extensions (#1689)

* docs: Document dual-catalog system for extensions

- Clarify distinction between catalog.json (curated) and catalog.community.json (reference)
- Update EXTENSION-DEVELOPMENT-GUIDE.md to explain community catalog submission
- Update EXTENSION-PUBLISHING-GUIDE.md with dual-catalog workflow
- Update EXTENSION-USER-GUIDE.md with catalog selection guidance
- Expand README.md with comprehensive catalog explanation
- Update RFC-EXTENSION-SYSTEM.md with dual-catalog design and current implementation
- Change GitHub references from statsperform to github
- Add SPECKIT_CATALOG_URL environment variable documentation

This clarifies how organizations can curate their own catalog while
browsing community-contributed extensions for discovery.

* Update extensions/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update extensions/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update extensions/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

extensions/EXTENSION-USER-GUIDE.md

@@ -76,13 +76,15 @@ vim .specify/extensions/jira/jira-config.yml
 
 ## Finding Extensions
 
+**Note**: By default, `specify extension search` uses your organization's catalog (`catalog.json`). If the catalog is empty, you won't see any results. See [Extension Catalogs](#extension-catalogs) to learn how to populate your catalog from the community reference catalog.
+
 ### Browse All Extensions
 
 ```bash
 specify extension search
 ```
 
-Shows all available extensions in the catalog.
+Shows all extensions in your organization's catalog.
 
 ### Search by Keyword
 
@@ -415,11 +417,15 @@ export SPECKIT_CATALOG_URL="https://example.com/staging/catalog.json"
 
 ---
 
+## Extension Catalogs
+
+For information about how Spec Kit's dual-catalog system works (`catalog.json` vs `catalog.community.json`), see the main [Extensions README](README.md#extension-catalogs).
+
 ## Organization Catalog Customization
 
-### Why the Default Catalog is Empty
+### Why Customize Your Catalog
 
-The default spec-kit catalog ships empty by design. This allows organizations to:
+Organizations customize their `catalog.json` to:
 
 - **Control available extensions** - Curate which extensions your team can install
 - **Host private extensions** - Internal tools that shouldn't be public