ci: set top-level permissions for antithesis-verify workflow

Add explicit `permissions: contents: read` at the workflow level to
restrict the default GITHUB_TOKEN to read-only access, following the
principle of least privilege.

This addresses the OpenSSF Scorecard Token-Permissions warning for
.github/workflows/antithesis-verify.yml.

Signed-off-by: Gagan H R <hrgagan4@gmail.com>

Author: gaganhr94

.github/workflows/antithesis-verify.yml

@@ -1,6 +1,9 @@
 ---
 name: Verify Antithesis Docker Compose Pipeline
 
+permissions:
+  contents: read
+
 on:
   push:
     branches: