ci: set explicit read permissions on main-ci workflow

[arpitjain099]

This narrows the GITHUB_TOKEN for .github/workflows/main-ci.yml to read-only.

• These jobs only check out the repo and run build/test steps, so contents: read covers them.
• Without an explicit block the token inherits the repo default, often write-enabled.
• Following the principle of least privilege from the GitHub Actions security hardening guide.

Behavior is unchanged.

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Latest commit: ff8f970

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