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, and the metadata export works on
a copy of the local index there. The README, ARCHITECTURE.md and
config.example.yml now say a backup needs free space there of the larger
of blob_size_limit and about three times the size of the local index.
Also corrected: the snapshot ID format, what restore reads and how
incomplete snapshots are removed in docs/DATAMODEL.md, what source_path
holds, 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.

Model: opus-5-5

TODO.md

@@ -22,6 +22,20 @@ 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, and the metadata export works
+  on a copy of the local index there. The README and
+  `config.example.yml` now say a backup needs free space there of the
+  larger of `blob_size_limit` and about three times the size of the
+  local index. Also corrected: the snapshot ID format, what restore
+  reads and how incomplete snapshots are removed in `docs/DATAMODEL.md`,
+  what `source_path` holds, 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 two messages say only what is true
   ([issue #240](https://git.eeqj.de/sneak/vaultik/issues/240)). A config
   file that others can read was warned about as containing S3