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

README.md

@@ -64,9 +64,8 @@ It was written to meet these requirements together:
 ## Getting started
 
 Installing needs Go 1.26.1 or later. `go install` builds the newest tagged
-version. `vaultik version` then reports that tag and calls the build a release.
-Generating a key pair needs `age-keygen`, which comes with
-[age](https://age-encryption.org/).
+version. `vaultik version` then reports that tag. Generating a key pair needs
+`age-keygen`, which comes with [age](https://age-encryption.org/).
 
 ```sh
 # Install vaultik.
@@ -294,7 +293,7 @@ list or a map prints as YAML.
 Sets one value in the config file. The key is a dotted path, as in
 `vaultik config set compression_level 9`.
 
-- Comments and formatting in the file are kept.
+- Comments in the file are kept. Blank lines can be removed.
 - A key that no setting has is refused. A key that is not in the file yet is
   created. A missing parent becomes a list when the next part of the path is a
   number, and a map otherwise.
@@ -307,12 +306,13 @@ The command refuses a change that would stop the config from loading. An example
 is `vaultik config set compression_level 99`. The file is then left unchanged.
 
 A config that does not load yet is checked less strictly. An example is the file
-`config init` writes, which has no `storage_url`. There the command refuses only
-three things:
+`config init` writes, which has no `storage_url`. There the command refuses
+these:
 
 - an unknown key
 - a value of the wrong type, such as a word where a number belongs
 - a `${ENV:NAME}` value whose variable is unset or empty
+- a value that is not a size, for a size setting
 
 ### `snapshot create`
 
@@ -1292,6 +1292,7 @@ destination still learns these:
 - when each backup finished
 - how many blobs each snapshot uses, and their sizes
 - which blobs two snapshots share
+- whether a blob holds data they already have in full
 - whether a hostname and snapshot name they guess exist there
 
 The last point holds because a remote key is a plain hash of the snapshot ID,
@@ -1503,8 +1504,8 @@ the version Go recorded in it:
 `v`, so the tag `v1.0.0` gives `vaultik 1.0.0`. `script/release-snapshot` stamps
 `dev-` followed by 12 characters of the commit.
 
-`vaultik version` calls a build a release only when its version is a plain tag,
-such as `v1.0.0` or `1.0.0`. Every other version is a development build.
+`vaultik version` adds a development-build note to every version except a plain
+tag, such as `v1.0.0` or `1.0.0`.
 
 `make` stops with an error when `script/version` gives no version.