feat: flatten vault structure — remove type-based folders
- scan_vault now scans root + system folders (type/, config/, theme/) only
- Type determined purely by frontmatter, no folder inference
- Remove move_note_to_type_folder (Rust command + frontend logic)
- Remove TYPE_FOLDER_MAP — new notes created at vault root
- Add migrate_to_flat_vault command (moves subfolder notes to root,
updates path-based wikilinks to title-based, cleans empty dirs)
- Migration runs automatically via Cmd+K > Repair Vault
- Update getting_started sample files for flat structure
- Update all tests (611 cargo + 2131 vitest passing)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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