List a missing file:// destination directory as an error (closes #220)

The file backend listed a destination directory that does not exist as
an empty store. With the volume unplugged, snapshot list reported every
local snapshot as missing from the store, snapshot remove said it had
removed metadata it never reached, and prune dropped every local
snapshot record. List and ListStream now fail when the destination
directory is missing, so those commands take their existing path for a
store that cannot be listed. A missing prefix under an existing
directory is still an empty listing, and a first backup still creates
the directory.

Three tests listed a file:// destination nothing had created; they now
create it.

Model: opus-5-5

TODO.md

@@ -22,6 +22,16 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-06: Made a `file://` destination whose directory is missing
+  count as one that cannot be listed
+  ([issue #220](https://git.eeqj.de/sneak/vaultik/issues/220)). The file
+  backend listed a missing directory as an empty store, so with the
+  quickstart's USB stick unplugged `snapshot list` reported every local
+  snapshot as missing from the store, `snapshot remove` claimed to have
+  removed metadata it never reached, and `prune` dropped every local
+  snapshot record. Listing a missing destination directory is now an
+  error; a first backup still creates the directory.
+
 - 2026-10-06: Made `--older-than` and `--keep-newer-than` reject a
   duration with characters outside its number-and-unit parts
   ([issue #215](https://git.eeqj.de/sneak/vaultik/issues/215)). The