add explicit permissions block to workflow

Addresses CodeQL security finding by restricting GITHUB_TOKEN
to read-only contents access.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dpark01

.github/workflows/validate-reference-genomes.yml

@@ -9,6 +9,9 @@ on:
       - 'assembly/reference_genomes.tsv'
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 jobs:
   validate:
     runs-on: ubuntu-latest