fix: add checks:write permission for security audit CI step

The rustsec/audit-check action needs checks:write to create
GitHub check runs. Without it: "Resource not accessible by integration"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Darth-Hidious

.github/workflows/rust-backbone.yml

@@ -16,6 +16,10 @@ on:
       - "crates/**"
       - ".github/workflows/rust-backbone.yml"
 
+permissions:
+  contents: read
+  checks: write
+
 concurrency:
   group: rust-backbone-${{ github.ref }}
   cancel-in-progress: true