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

A blob is written whole to a temporary file and uploaded once finished,
not streamed to storage. The README, ARCHITECTURE.md and
config.example.yml now say a backup needs free temporary space for each
blob (twice that for an rclone destination that cannot stream uploads)
and for the metadata export's copies of the local index, in $TMPDIR, or
partly in /var/tmp when TMPDIR is unset. 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.