ci: add GitHub Actions workflow for automated testing

Add CI pipeline that runs on push to main, pull requests, and manual
trigger. The workflow sets up the required build environment (Java 21,
Python 3.12, Gradle) and executes the full test suite including:

- Syncing addon source repositories
- Downloading latest release fixture jars
- Running Gradle tests with a 45-minute timeout
- Uploading test reports and release summaries as artifacts

This ensures code quality and catches regressions early in the
development cycle.

Author: GhostTypes

.github/workflows/ci.yml

@@ -0,0 +1,75 @@
+name: CI
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Download Fixtures And Run Tests
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Java 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "21"
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Ensure Gradle wrapper is executable
+        run: chmod +x gradlew
+
+      - name: Sync addon source repos
+        run: python tools/download_latest_release_jars.py --update
+
+      - name: Download latest release fixture jars
+        run: ./gradlew downloadLatestReleaseJars --no-daemon
+
+      - name: Run test suite
+        run: ./gradlew test --no-daemon
+
+      - name: Upload release summaries
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: release-summaries
+          if-no-files-found: warn
+          path: |
+            ai_reference/addons/clone-summary.json
+            fixtures/addons/jars/release-summary.json
+            fixtures/addons/jars/release-summary.csv
+            fixtures/addons/jars/release-summary.txt
+
+      - name: Upload test reports
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-reports
+          if-no-files-found: warn
+          path: |
+            build/reports/tests/test/**
+            build/test-results/test/**