Add Cleanup Extension to README (#1678)

Author: dsrednicki

extensions/README.md

@@ -7,6 +7,7 @@ Community-contributed extensions for [Spec Kit](https://github.com/github/spec-k
 | Extension | Purpose | URL |
 |-----------|---------|-----|
 | V-Model Extension Pack | Enforces V-Model paired generation of development specs and test specs with full traceability | [spec-kit-v-model](https://github.com/leocamello/spec-kit-v-model) |
+| Cleanup Extension | Post-implementation quality gate that reviews changes, fixes small issues (scout rule), creates tasks for medium issues, and generates analysis for large issues | [spec-kit-cleanup](https://github.com/dsrednicki/spec-kit-cleanup) |
 
 ## Adding Your Extension