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/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.