RFC 0003: Epic / Tracking issue for Google native file export

[rmanibus]

Context

Track completion and rollout for RFC 0003 (Google Native File Export).

RFC: rfcs/0003-google-native-file-export.md

Goal

Export Google-native documents (Docs, Sheets, Slides, etc.) during backup into deterministic file representations so snapshots include meaningful content for those items.

Scope

• Extend Google Drive source scanning/reading to detect native document MIME types.
• Export native files through supported Google export formats.
• Integrate exported content into backup scan flow.
• Add tests for native export behavior and edge cases.

Linked PR

• feat: Google native file export (RFC 0003) #72

[rmanibus]

Implementation PR: #72

[rmanibus]

Related RFC PR: #64