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

internal/cli/entry_banner_test.go

@@ -172,9 +172,9 @@ func TestBannerSuppressedInArgs(t *testing.T) {
 
 // hermeticConfig is a complete, valid config that needs no network and
 // no credentials: file:// storage is exempt from the S3 credential
-// checks, and FileStorer over a directory that does not exist lists
-// zero objects without erroring. Chunk, blob and compression settings
-// are filled in by config.Load.
+// checks. A test that lists the destination must create its directory
+// first, because listing a directory that does not exist is an error.
+// Chunk, blob and compression settings are filled in by config.Load.
 const hermeticConfig = `age_recipients:
   - age1278m9q7dp3chsh2dcy82qk27v047zywyvtxwnj4cvt0z65jw6a7q5dqhfj
 snapshots:
@@ -196,22 +196,25 @@ hostname: test-host
 // `snapshot list` is the command chosen because it is the only --json
 // command that reaches its document without a populated destination
 // store: it reads the local index, streams `metadata/` (empty here),
-// and treats a barren destination as an empty list rather than a
-// failure.
+// and treats an empty destination directory as an empty list rather
+// than a failure.
 //
 // Not parallel: it replaces os.Args, os.Stdout and the xdg globals.
 func TestEntryJSONStdoutIsExactlyOneDocument(t *testing.T) {
 	dir := t.TempDir()
 	configPath := filepath.Join(dir, "config.yml")
+	storeDir := filepath.Join(dir, "store")
 
 	contents := fmt.Sprintf(hermeticConfig,
 		filepath.Join(dir, "source"),
-		filepath.Join(dir, "store"),
+		storeDir,
 		filepath.Join(dir, "index.sqlite"))
 
 	require.NoError(t,
 		os.WriteFile(configPath, []byte(contents), configFileMode))
 
+	require.NoError(t, os.Mkdir(storeDir, 0o750))
+
 	// 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.