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

config.example.yml

@@ -295,8 +295,10 @@ storage_url: "rclone://myremote/path/to/backups"
 
 # Path to local SQLite index database
 # This database tracks file state for incremental backups
-# Default: /var/lib/vaultik/index.sqlite
-#index_path: /var/lib/vaultik/index.sqlite
+# Default: the platform data directory, e.g.
+#   macOS: ~/Library/Application Support/vaultik/index.sqlite
+#   Linux: ~/.local/share/vaultik/index.sqlite
+#index_path: /path/to/index.sqlite
 
 # Average chunk size for content-defined chunking
 # Smaller chunks = better deduplication but more metadata
@@ -311,6 +313,12 @@ storage_url: "rclone://myremote/path/to/backups"
 # Chunking uses no secret (the FastCDC parameters are fixed and public). At a
 # large limit a blob holds hundreds of chunks, so individual chunk lengths are
 # not visible in its size; lowering the limit toward chunk_size exposes them.
+# A backup needs free temporary space, because each blob is written whole to
+# a temporary file before it is uploaded (up to about blob_size_limit; an
+# rclone destination that cannot stream uploads needs about twice that) and
+# the metadata export writes copies of the local index. Temporary files go to
+# $TMPDIR (default /tmp); with TMPDIR unset, SQLite writes one of those copies
+# to /var/tmp.
 # Supports: 1GB, 10G, 500MB, 1GiB, etc.
 # Default: 10GB
 #blob_size_limit: 10GB