Drop the internal working database from the README

It's an implementation detail the app deliberately hides: the user names and
moves a JSON file, and nothing else. Documenting the derived SQLite copy
invited people to think about (or back up, or move) a file that is rebuilt on
demand. The rationale for keeping it in private storage stays where it belongs,
in sync.rs's module docs.

README.md

@@ -14,18 +14,14 @@ src-tauri/           Tauri v2 shell — exposes the core as typed commands
 crates/mtgc-core/    Pure Rust: db, fuzzy search, filters, decks, ingester
 ```
 
-One file you manage, and two you don't:
+Two files:
 
 - **your library `.json`** — owned cards, decks, deck contents. Small,
-  readable, **the only file you name or move**. See [Sync](#sync).
-- **`working.db`** — internal SQLite working copy of that document. Rebuilt
-  whenever needed; nothing is lost if you delete it.
-- **`reference.db`** — read-only card data built from Scryfall bulk. Bulky,
-  re-downloaded per device, **never synced**.
-
-The two databases are joined at runtime via `ATTACH` and both live in the OS
-app-data dir (e.g. `~/.local/share/com.garrettmills.mtgc/`). The library
-document lives wherever you put it.
+  readable, and the only file you name or move. Put it somewhere synced; see
+  [Sync](#sync).
+- **`reference.db`** — read-only card data built from Scryfall bulk. Bulky and
+  re-downloaded per device, so it's never synced. Lives in the OS app-data dir
+  (e.g. `~/.local/share/com.garrettmills.mtgc/`).
 
 ## First-time setup
 
@@ -265,10 +261,6 @@ storage permission at all** — no "All files access" prompt. The alternative,
 `MANAGE_EXTERNAL_STORAGE`, would hand the app the entire shared volume just to
 reach one directory.
 
-The working database stays in app-private storage regardless: SQLite can't open
-a `content://` URI, and a database on a sync-managed filesystem risks torn
-writes. Only the document crosses that boundary.
-
 ## Branding
 
 The logo is an original pirate-ship silhouette in the terracotta brand color