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

.github/workflows/release.yml

@@ -0,0 +1,39 @@
+name: Release
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+
+      - uses: gradle/actions/setup-gradle@v4
+
+      - name: Decode keystore
+        run: echo "${{ secrets.KEYSTORE_BASE64 }}" | base64 -d > ${{ runner.temp }}/keystore.jks
+
+      - name: Build signed release APK
+        env:
+          KEYSTORE_PATH: ${{ runner.temp }}/keystore.jks
+          KEYSTORE_PASSWORD: ${{ secrets.KEYSTORE_PASSWORD }}
+          KEY_ALIAS: ${{ secrets.KEY_ALIAS }}
+          KEY_PASSWORD: ${{ secrets.KEYSTORE_PASSWORD }}
+        run: ./gradlew :androidApp:assembleRelease
+
+      - name: Upload release APK
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          mv androidApp/build/outputs/apk/release/androidApp-release.apk nikki-${{ github.event.release.tag_name }}.apk
+          gh release upload ${{ github.event.release.tag_name }} nikki-${{ github.event.release.tag_name }}.apk