[extension] Add Golden Demo extension to community catalog (#3151)

* Add Golden Demo extension to community catalog

Add golden-demo extension submitted by @jasstt to:
- extensions/catalog.community.json (alphabetical order)
- docs/community/extensions.md community extensions table

Closes #3127

Assisted-by: GitHub Copilot (model: claude-sonnet-4.6, autonomous)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove empty changelog field from golden-demo catalog entry

Assisted-by: GitHub Copilot (model: claude-sonnet-4.6, autonomous)

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

extensions/catalog.community.json

@@ -1,6 +1,6 @@
 {
   "schema_version": "1.0",
-  "updated_at": "2026-06-23T00:00:00Z",
+  "updated_at": "2026-06-24T00:00:00Z",
   "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.community.json",
   "extensions": {
     "aide": {
@@ -1327,6 +1327,39 @@
       "created_at": "2026-04-12T15:30:00Z",
       "updated_at": "2026-04-13T14:39:00Z"
     },
+    "golden-demo": {
+      "name": "Golden Demo",
+      "id": "golden-demo",
+      "description": "Extracts acceptance criteria from specs, builds test vectors, and produces a behavioral drift report — complementary to Architecture Guard and CDD.",
+      "author": "jasstt",
+      "version": "0.1.1",
+      "download_url": "https://github.com/jasstt/spec-kit-golden-demo/archive/refs/tags/v0.1.1.zip",
+      "repository": "https://github.com/jasstt/spec-kit-golden-demo",
+      "homepage": "https://github.com/jasstt/spec-kit-golden-demo",
+      "documentation": "https://github.com/jasstt/spec-kit-golden-demo",
+      "license": "MIT",
+      "category": "docs",
+      "effect": "read-write",
+      "requires": {
+        "speckit_version": ">=0.1.0"
+      },
+      "provides": {
+        "commands": 2,
+        "hooks": 2
+      },
+      "tags": [
+        "testing",
+        "drift-detection",
+        "behavioral-oracle",
+        "tdd",
+        "quality"
+      ],
+      "verified": false,
+      "downloads": 0,
+      "stars": 0,
+      "created_at": "2026-06-24T00:00:00Z",
+      "updated_at": "2026-06-24T00:00:00Z"
+    },
     "harness": {
       "name": "Research Harness",
       "id": "harness",