Wire keystore signing config; trim publish.md

- Read keystore.properties at Gradle config time; wire signingConfig into
  the release build type (degrades gracefully when the file is absent)

SchwertUndMagieOnPebbleCompanionApp/app/build.gradle.kts

@@ -4,6 +4,11 @@ plugins {
     alias(libs.plugins.android.application)
 }
 
+val keystoreProps = Properties().apply {
+    rootProject.file("keystore.properties").takeIf { it.exists() }
+        ?.reader()?.use { load(it) }
+}
+
 android {
     namespace = "de.ladkau.schwertundmagieonpebblecompanionapp"
     compileSdk {
@@ -35,8 +40,18 @@ android {
         }
     }
 
+    signingConfigs {
+        create("release") {
+            keystoreProps["storeFile"]?.let { storeFile = rootProject.file(it as String) }
+            storePassword = keystoreProps["storePassword"] as String?
+            keyAlias      = keystoreProps["keyAlias"]     as String?
+            keyPassword   = keystoreProps["keyPassword"]  as String?
+        }
+    }
+
     buildTypes {
         release {
+            signingConfig = signingConfigs.getByName("release")
             isMinifyEnabled = false
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),