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)

.github/workflows/build.yml

@@ -0,0 +1,31 @@
+name: Build & Test
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+
+      - uses: gradle/actions/setup-gradle@v4
+
+      - name: Run tests
+        run: ./gradlew :composeApp:testDebugUnitTest
+
+      - name: Build debug APK
+        run: ./gradlew :androidApp:assembleDebug
+
+      - name: Upload debug APK
+        uses: actions/upload-artifact@v4
+        with:
+          name: debug-apk
+          path: androidApp/build/outputs/apk/debug/*.apk