Start empty when the cache directory holds another account's cache (closes #104)

When metadata.json in the cache directory was written for a different,
non-zero user ID than the client's, Library.open deletes it and mldata/
and loads an empty store, so the first refresh enumerates from 0 and none
of the other account's collections, files, keys or ML results are served.
Only reachable with --cache-dir or an explicit cacheDirectory.

Model: opus-5-5

TODO.md

@@ -18,12 +18,17 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Kept one account's cache from mixing with another's (issue 104).
+  When `metadata.json` in the cache directory was written for a different,
+  non-zero user ID than the client's, `Library.open` deletes it and `mldata/`
+  and starts empty, so the first refresh enumerates from 0. This only happens
+  with `--cache-dir` or an explicit `cacheDirectory`; the default path already
+  includes the user ID. A test opens one account's cache as another account.
 - 2026-09-23: `backup-metadata` no longer stops on one failed ML data request
   (issue 101). Each request of up to 200 files is tried on its own; a failed one
   is logged, its files are written with the reason in `mlDataError`, and the
   command exits 1 once the dump is complete. `fetchMLData`, which only this
   command used, is gone; the command calls `fetchMLDataBatch` per batch.
-
 - 2026-09-23: Single-sourced the version string (issue 5). `package.json` is the
   only place it is written: `src/index.ts` imports it for `VERSION` and
   `bin/quak.ts` passes `VERSION` to commander. tsc copies `package.json` to