ci: cache Gradle, harden permissions, fix test triggers (#45)

- Add gradle/actions/setup-gradle caching to all three workflows
- test.yml: trigger on pull_request + push to master (was branches-ignore: master), so PRs from forks are covered and master is verified after merge; add least-privilege permissions and PR-only concurrency
- publishRelease.yml: drop unused 'secrets: inherit' and the dead SNAPSHOT env var (Gradle reads the snapshot project property, not a plain env var); add contents: read permissions; fix the misleading Maven Central comment (upload only stages on Central Portal, the final Publish is manual)
- docs.yml: add Gradle caching

.github/workflows/test.yml

@@ -2,9 +2,17 @@ name: Test
 
 on:
   push:
-    branches-ignore: [master]
+    branches: [master]
+  pull_request:
   workflow_call:
 
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   test:
     name: ${{ matrix.name }}
@@ -27,5 +35,7 @@ jobs:
         with:
           distribution: temurin
           java-version: '21'
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
       - name: Test
         run: ./gradlew ${{ matrix.tasks }} --console=plain