Add optional Google Analytics integration

[walishcs]

Summary

• add an optional GA4 measurement ID to Keystatic Site settings
• link to Google’s official instructions for finding the tag ID
• load the Google tag only in production and rely on GA4 history-change measurement for Astro page transitions
• keep empty or invalid IDs from injecting analytics
• document the production-only analytics rule in AGENTS.md

Impact

Editors can configure a public GA4 measurement ID locally through Keystatic and publish it with site content. Local development does not send analytics data. Existing Site settings files remain valid when the new field is absent.

Validation

• npm run check
• npm test (43 tests)
• npm run format:check
• npm run build
• npm run astryx -- doctor
• Keystatic UI verification for the field and official help link