feat: Google native file export (RFC 0003)

[rmanibus]

Summary

Implement RFC 0003 by exporting Google-native files (Docs/Sheets/Slides/etc.) during backup so they are represented as real content in snapshots.

What Changes

• Google Drive source enhancements:
  • detect native document MIME types
  • add export handling for native files
  • wire export path into normal source walk/change flows
• Backup scan integration updates for native export handling.
• CLI backup flow updates related to Google native handling.
• Tests added/expanded for native export behavior and backup scan integration.

Files of Interest

• pkg/source/gdrive_native.go
• pkg/source/gdrive_native_test.go
• pkg/source/gdrive.go
• pkg/source/gdrive_changes.go
• internal/engine/backup_scan.go
• internal/engine/backup_scan_test.go
• pkg/source/gdrive_test.go

Behavior Notes

• Google-native documents are exported into supported formats before content-addressed storage.
• Non-native files continue through the existing download path.

Tracking

• Story / epic: RFC 0003: Epic / Tracking issue for Google native file export #99
• RFC: rfcs/0003-google-native-file-export.md
• Milestone: RFC 0003: Google native file export

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 36 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/source/gdrive.go	40.00%	24 Missing ⚠️
cmd/cloudstic/cmd_backup.go	33.33%	8 Missing ⚠️
pkg/source/gdrive_changes.go	33.33%	4 Missing ⚠️

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[Copilot]

Pull request overview

This PR implements RFC 0003 by adding support for exporting Google-native files (Docs, Sheets, Slides, etc.) during backup. Native files, which cannot be downloaded directly from Google Drive, are now exported to standard formats (e.g., .docx, .xlsx, .pptx) before content-addressed storage. Change detection uses headRevisionId instead of size/hash comparisons, which are unreliable for exported files.

Changes:

• New gdrive_native.go with MIME type detection, export format mapping, and extension helpers; integrated into Walk, WalkChanges, toFileMeta, and GetFileStream flows.
• Change detection in backup_scan.go uses headRevisionId-based fast path for native files, carrying forward content metadata when unchanged.
• CLI --skip-native-files flag to opt out of native file export for both gdrive and gdrive-changes source types.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rfcs/0003-google-native-file-export.md	Status updated to Implemented; section renumbered
pkg/source/gdrive_native.go	New file: native MIME detection, export MIME/extension mapping
pkg/source/gdrive_native_test.go	Tests for native helper functions
pkg/source/gdrive.go	GDriveSource gains mimeTypes map, skipNativeFiles option, export routing in GetFileStream, native name/metadata handling in toFileMeta and visitEntryWithPath
pkg/source/gdrive_changes.go	WalkChanges initializes mimeTypes, adds headRevisionId to fields, skips native files when opted out, records MIME in changeToFileChange
pkg/source/gdrive_test.go	Comprehensive tests for toFileMeta, visitEntryWithPath, changeToFileChange, and option wiring
internal/engine/backup_scan.go	detectChange native fast-path using headRevisionId; new isGoogleNativeMeta helper
internal/engine/backup_scan_test.go	Tests for native change detection, empty revID, and metadata carry-forward
cmd/cloudstic/cmd_backup.go	--skip-native-files CLI flag; initSource updated for both gdrive source types

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