Correct doc and help sentences that are false about the code (closes #233)

A blob is written in full to a temporary file in $TMPDIR and uploaded
once finished, not streamed to storage; the README and
config.example.yml now say a backup needs about blob_size_limit of free
space there. Also corrected: the snapshot ID format, what restore reads
and how incomplete snapshots are removed in docs/DATAMODEL.md, what
source_path is for, the index_path default, the config search order in
the snapshot create help, what snapshot remove cleans up in the prune
help, how the release installs Go, and the script/release and
script/fmt-check comments.

The source_path claim is also corrected in models.go, scanner.go and
001.sql, which the issue did not list.

Model: opus-5-5

TODO.md

@@ -22,6 +22,18 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Corrected documentation, help text and comments that were
+  false about the code
+  ([issue #233](https://git.eeqj.de/sneak/vaultik/issues/233)). A blob
+  is not streamed to storage; it is written in full to a temporary file
+  in `$TMPDIR` and uploaded once finished, so the README and
+  `config.example.yml` now say a backup needs about `blob_size_limit` of
+  free space there. Also corrected: the snapshot ID format, what restore
+  reads and how incomplete snapshots are removed in `docs/DATAMODEL.md`,
+  what `source_path` is for, the `index_path` and config file defaults,
+  what `snapshot remove` cleans up, how the release gets its Go
+  toolchain, and the `script/release` and `script/fmt-check` comments.
+
 - 2026-10-07: Made `s3.part_size` set the multipart upload part size
   ([issue #232](https://git.eeqj.de/sneak/vaultik/issues/232)). It was
   loaded and defaulted but never passed to the S3 client, whose uploader