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/types/types.go

@@ -155,8 +155,8 @@ type BlobHash string
 // FilePath represents an absolute path to a file or directory.
 type FilePath string
 
-// SourcePath represents the root directory from which files are backed up.
-// Used during restore to strip the source prefix from paths.
+// SourcePath is the source directory a scan found a file under, made
+// absolute and with symlinks resolved.
 type SourcePath string
 
 // Hostname identifies a host machine.