fix: add explicit permissions to CI workflow

[bushidocodes]

Summary

• Adds permissions: contents: read at the top-level of .github/workflows/ci.yml
• This applies least-privilege token scope to both the notebook and shell-lint jobs, which only need to checkout code

Why

GitHub's code scanning flagged two open actions/missing-workflow-permissions warnings (alerts #3 and #4). Without explicit permissions, workflow jobs inherit the default GITHUB_TOKEN scope, which is broader than necessary and violates least-privilege best practice.

Reviewer notes

No functional change — only the token permissions are narrowed. Both jobs only call actions/checkout and run local scripts, so contents: read is the correct minimal scope.

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