Add CI/CD workflows and release signing config

Build workflow: runs tests + assembles debug APK on pull requests.
Release workflow: builds signed APK and uploads to GitHub release.
Signing config reads keystore from env vars (set by CI secrets).
Keystore files excluded from git.

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>
Co-Authored-By: Claude Opus 4.6 (1M context)

androidApp/build.gradle.kts

@@ -16,6 +16,18 @@ android {
         versionName = "1.0.0"
     }
 
+    signingConfigs {
+        create("release") {
+            val path = System.getenv("KEYSTORE_PATH")
+            if (path != null) {
+                storeFile = file(path)
+                storePassword = System.getenv("KEYSTORE_PASSWORD")
+                keyAlias = System.getenv("KEY_ALIAS")
+                keyPassword = System.getenv("KEY_PASSWORD")
+            }
+        }
+    }
+
     buildTypes {
         release {
             isMinifyEnabled = true
@@ -23,6 +35,7 @@ android {
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"
             )
+            signingConfig = signingConfigs.getByName("release")
         }
     }