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

internal/cli/snapshot.go

@@ -66,8 +66,9 @@ func newSnapshotCreateCommand() *cobra.Command {
 If snapshot names are provided, only those snapshots are created.
 If no names are provided, all configured snapshots are created.
 
-Config is located at /etc/vaultik/config.yml by default, but can be overridden by
-specifying a path using --config or by setting VAULTIK_CONFIG to a path.`,
+The config is read from the path given by --config or VAULTIK_CONFIG;
+otherwise from the platform config directory (~/.config/vaultik/config.yml
+on Linux), then /etc/vaultik/config.yml.`,
 		Args: cobra.ArbitraryArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			// Pass snapshot names from args