[StepSecurity] ci: Harden GitHub Actions

Signed-off-by: StepSecurity Bot <bot@stepsecurity.io>

Author: step-security-bot

.github/workflows/main-ci.yml

@@ -6,10 +6,18 @@ on:
   pull_request:
     branches: [ "main" ]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest
     steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@v2
+        with:
+          egress-policy: audit
+
       - uses: actions/checkout@v4
       - name: Set up JDK 17
         uses: actions/setup-java@v4