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

docs/DATAMODEL.md

@@ -195,8 +195,9 @@ example is `server1_home_2025-06-01T12:00:00Z`. It has three parts:
 The `snapshot list` table shows `uncompressed_chunk_size` as `UNCOMPRESSED SIZE`
 and `new_chunk_size` as `NEW CHUNK SIZE`.
 
-A snapshot with `completed_at` set to `NULL` is unfinished. `snapshot list` does
-not show it. The next backup deletes it, and so does `prune`.
+A snapshot with `completed_at` set to `NULL` is unfinished. `snapshot list`
+shows it only when its manifest is at the destination, and then as remote only.
+The next backup deletes it, and so does `prune`.
 
 ### `snapshot_files`
 
@@ -324,7 +325,8 @@ Storing a changed file writes these rows:
 
 - A row in `blobs`, when packing of a blob starts.
 - When the blob is closed, in one transaction:
-    - a row in `chunks` for each new chunk in the blob
+    - a row in `chunks` for each chunk queued since the last blob was closed.
+      When a blob is full, that includes the first chunk of the next blob.
     - the blob's rows in `blob_chunks`
     - the blob's `blob_hash`, sizes and `finished_ts`
 - When the blob is stored at the destination, in one transaction: