release: bump versionCode 5->6 for v0.4.4 + add changelog 6.txt

v0.4.3 already shipped as versionCode 5; a duplicate code would be rejected by Play and ignored by F-Droid. Bump to 6 unblocks the v0.4.4 release. QA gate passed (units + on-device E2E + visual render check screenshots in docs/qa/render-check/).

android/app/build.gradle

@@ -94,7 +94,7 @@ android {
         applicationId 'cc.agentlabs.opencode'
         minSdkVersion rootProject.ext.minSdkVersion
         targetSdkVersion rootProject.ext.targetSdkVersion
-        versionCode 5
+        versionCode 6
         versionName "0.4.4"
 
         buildConfigField "String", "REACT_NATIVE_RELEASE_LEVEL", "\"${findProperty('reactNativeReleaseLevel') ?: 'stable'}\""