feat: title = filename — wikilink resolution + slug collision detection

- Wikilink resolution order: filename stem (primary) → alias → title (fallback)
- Slug collision detection: auto-suffix (-2, -3, etc.) on note creation
- Add slugCollides utility for frontend collision checking
- resolveNewNote accepts entries param for collision-aware creation
- Title editing via H1 heading already triggers file rename (existing flow)
- Update docs/ARCHITECTURE.md with migrate_to_flat_vault command

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -569,7 +569,7 @@ The vault backend (`src-tauri/src/vault/`) is split into focused submodules:
 | `trash.rs` | `purge_trash` — deletes trashed notes older than 30 days |
 | `rename.rs` | `rename_note` — renames files and updates wikilinks across the vault |
 | `image.rs` | `save_image` — saves base64-encoded attachments with sanitized filenames |
-| `migration.rs` | Frontmatter migration utilities |
+| `migration.rs` | Frontmatter migration: `migrate_is_a_to_type`, `migrate_to_flat_vault` |
 | `config_seed.rs` | Seeds `config/` folder, migrates `AGENTS.md`, repairs missing config files |
 | `getting_started.rs` | Creates the Getting Started demo vault |
 
@@ -611,6 +611,7 @@ The vault backend (`src-tauri/src/vault/`) is split into focused submodules:
 | `purge_trash` | Delete notes trashed >30 days ago |
 | `reload_vault` | Invalidate cache and full rescan from filesystem → `Vec<VaultEntry>` |
 | `reload_vault_entry` | Re-read a single file from disk → `VaultEntry` |
+| `migrate_to_flat_vault` | Move notes from type subfolders to vault root, update wikilinks |
 | `check_vault_exists` | Check if vault path exists |
 | `create_getting_started_vault` | Bootstrap demo vault |