Publish Vulpes Preview 15 sources for desktop, Android and Tundra

Share Gaia assets and adapters, keep private profiles and development documents local, and update both public READMEs.

release.json

@@ -0,0 +1,17 @@
+{
+  "gaia": "2.7.0",
+  "channel": "preview",
+  "revision": 15,
+  "androidVersionCode": 15,
+  "geckoview": "156.0.20260921121718",
+  "engineAlignment": {
+    "family": "156",
+    "patchParity": false,
+    "reason": "Firefox desktop 156.0.1; Mozilla's GeckoView release repository currently offers 156.0 only. Keep the desktop patch and report actual engine versions."
+  },
+  "version": "2.7-preview.15",
+  "adapters": {
+    "desktop": "0.1.0-dev",
+    "android": "0.1.0-dev"
+  }
+}