fix(ci): pass workflow_run head_branch through env; read-only tokens

auto-merge.yml interpolated ${{ github.event.workflow_run.head_branch }} straight into a run script that holds a contents:write token. A fork pull request controls that branch name, so it could run commands with write access (CodeQL actions/code-injection/critical). It now arrives as an environment variable.

ci.yml, code-quality.yml and deploy.yml get permissions: contents: read; none of them writes through GITHUB_TOKEN.

.github/workflows/code-quality.yml

@@ -6,6 +6,11 @@ on:
   pull_request:
     branches: [main]
 
+
+# Read-only token: these jobs never write through GITHUB_TOKEN.
+permissions:
+  contents: read
+
 jobs:
   lint:
     name: ESLint