fix: use live Pages URL for social metadata

[atharvmantri]

Summary

• derive the public site URL once from the Pages build mode
• use that URL for metadataBase and Open Graph metadata
• keep preview/custom-domain behavior unchanged outside Pages builds

Validation

• MEMORYGUARD_GITHUB_PAGES=true pnpm --filter @memoryguard/landing build
• Pages-mode output contains the live GitHub Pages URL in og:url
• Pages-mode output contains no stale memoryguard.atharv.me URL
• git diff --check

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