Fix the six findings of the review on config set, version and blob names

README.md: config set keeps comments and can remove blank lines; a
config that does not load yet also has a value that is not a size
refused; vaultik version reports a tag and adds a development-build
note to every other version. docs/REPOSTRUCTURE.md and README.md:
someone who has a blob's whole contents can compute its name and see
whether it is stored. docs/DATAMODEL.md: snapshot list shows an
unfinished snapshot as remote only once its manifest is at the
destination; closing a full blob commits the first chunk of the next
one. internal/cli/config.go: config set --help says the same about
comments and blank lines.

Model: fable-5-1

internal/cli/config.go

@@ -446,9 +446,9 @@ func newConfigSetCommand() *cobra.Command {
 		Use:   "set <key> <value>",
 		Short: "Set a config value by dotted path (e.g. compression_level 5)",
 		Long: `Sets a scalar config value addressed by dotted YAML path and writes
-the file back, preserving comments and formatting. Missing keys along
-the path are created: as a list when the next path element is a
-number, otherwise as a map.
+the file back. Comments in the file are kept. Blank lines can be
+removed. Missing keys along the path are created: as a list when the
+next path element is a number, otherwise as a map.
 
 If the config loads, a change after which it would not, such as
 compression_level 99, is refused and the file is left unchanged. A