- Simplify flatten_vault API to return usize instead of MigrationResult struct
- Add KEEP_FOLDERS: attachments/ and _themes/ alongside type/, config/, theme/
- Use HashSet for collision tracking in unique_filename
- Update wikilinks from path-based [[folder/slug]] to title-based [[slug]]
- Clean up empty directories after flattening
- Flatten demo-vault-v2: move all notes from type-based subfolders to root
- Update smoke tests for flat vault structure
- Remove migrate_to_flat_vault from repair_vault (one-time migration only)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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