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.

Two test fixtures listed a file:// destination nothing had created; they
now create it.

Model: opus-5-5

internal/cli/entry_prune_json_test.go

@@ -98,25 +98,30 @@ func TestEntryPruneJSONStdoutIsExactlyOneDocument(t *testing.T) {
 	}
 }
 
-// writeHermeticPruneConfig builds a config over a temp directory and, if
-// seedStale is set, creates the index database up front with one
-// snapshot record that has no counterpart on the destination store.
-// Returns the config path.
+// writeHermeticPruneConfig builds a config over a temp directory with an
+// empty destination directory and, if seedStale is set, creates the
+// index database up front with one snapshot record that has no
+// counterpart on the destination store. Returns the config path.
 func writeHermeticPruneConfig(t *testing.T, seedStale bool) string {
 	t.Helper()
 
 	dir := t.TempDir()
 	configPath := filepath.Join(dir, "config.yml")
 	indexPath := filepath.Join(dir, "index.sqlite")
+	storeDir := filepath.Join(dir, "store")
 
 	contents := fmt.Sprintf(hermeticConfig,
 		filepath.Join(dir, "source"),
-		filepath.Join(dir, "store"),
+		storeDir,
 		indexPath)
 
 	require.NoError(t,
 		os.WriteFile(configPath, []byte(contents), configFileMode))
 
+	// prune fails on a destination directory that does not exist, so the
+	// empty store is created here rather than left to a first backup.
+	require.NoError(t, os.Mkdir(storeDir, 0o755))
+
 	// The PID lock lives under xdg.DataHome, which xdg resolves at
 	// package init; point it at the temp dir so the test neither
 	// touches nor collides with the real one.