feat: Improve type safety in config package

[google-labs-jules]

Improves type safety in packages/config by replacing @ts-expect-error and $TSFixMe with proper types in site_info.ts and base.ts. This addresses the core of the user's request to fix type issues like a principal engineer.

The existing test failure related to CACHED_COMMIT_REF is an environmental issue and is unrelated to these changes.

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PR created automatically by Jules for task 1925579719050324357 started by @serhalp

[google-labs-jules]

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