Fix nine review findings in the documents and help text

- snapshot verify and snapshot restore also list the destination for a
  snapshot ID that is not in the local index: README, REPOSTRUCTURE,
  ARCHITECTURE, DATAMODEL and the two commands' help.
- prune on a separate machine deletes only unused data and does not
  remove old snapshots.
- The --quiet usage string names everything the flag leaves out.
- A warning written as a log record stays on stderr.
- config set on a config that does not load refuses three things.
- No test is held back by a build tag.
- A plain docker build . adds no -dirty.
- The backup's order of work records directories.
- TODO.md says what parallel downloads would speed up and names
  restorePlan and restoreSweeper.

Model: fable-5-1

internal/cli/root.go

@@ -58,8 +58,9 @@ an rclone remote. The backed-up machine needs no private key: only
 	cmd.PersistentFlags().BoolVar(&rootFlags.Debug, "debug", false,
 		"Enable debug output")
 	cmd.PersistentFlags().BoolVarP(&rootFlags.Quiet, "quiet", "q", false,
-		"Suppress status output (warnings, errors, JSON documents, "+
-			"'config get' values and confirmation prompts are still printed)")
+		"Leave out the banner, status lines, tables and reports (warnings, "+
+			"errors, JSON documents, 'config get' values and confirmation "+
+			"prompts are still printed)")
 	cmd.PersistentFlags().BoolVar(&rootFlags.SkipErrors, "skip-errors", false,
 		"Skip files that cannot be read when creating a snapshot, or "+
 			"that cannot be restored when restoring, instead of aborting "+

internal/cli/snapshot.go

@@ -252,7 +252,8 @@ snapshot's encrypted database is present. It does not read blob
 contents and does not need the age private key.
 
 Needs permission to read the backup destination store, and to list it
-when a remote key is given.
+when given a remote key, or a snapshot ID that is not in the local
+index.
 
 Use --deep to download, decrypt, and re-hash every blob to detect
 corruption -- integrity, not who wrote it.

internal/cli/snapshot_restore.go

@@ -49,8 +49,9 @@ typing it, so that it does not land in your shell history:
   export VAULTIK_AGE_SECRET_KEY="$(cat vaultik_backup_private_key.txt)"
 
 Also needs permission to read the backup destination store (and to list
-it when a remote key is given), and free space in the temporary directory
-for the decrypted blobs.
+it when given a remote key, or a snapshot ID that is not in the local
+index), and free space in the temporary directory for the decrypted
+blobs.
 
 Examples:
   # Restore entire snapshot