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; the README and
config.example.yml now say a backup needs about blob_size_limit of free
space there. Also corrected: the snapshot ID format, what restore reads
and how incomplete snapshots are removed in docs/DATAMODEL.md, what
source_path is for, 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.

The source_path claim is also corrected in models.go, scanner.go and
001.sql, which the issue did not list.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -58,8 +58,8 @@ type Scanner struct {
 	compressionLevel int
 	ageRecipient     string
 	snapshotID       string // Current snapshot being processed
-	// currentSourcePath is the source directory being scanned (used for
-	// restore path stripping).
+	// currentSourcePath is the source directory being scanned, stored with
+	// each file record.
 	currentSourcePath string
 	exclude           []string          // Glob patterns for files/directories to exclude
 	compiledExclude   []compiledPattern // Compiled glob patterns
@@ -211,7 +211,6 @@ func (s *Scanner) Scan(
 	ctx context.Context, path string, snapshotID string,
 ) (*ScanResult, error) {
 	s.snapshotID = snapshotID
-	// Store source path for file records (used during restore)
 	s.currentSourcePath = path
 	result := &ScanResult{
 		StartTime: time.Now().UTC(),
@@ -1198,9 +1197,8 @@ func (s *Scanner) checkFileInMemory(
 	}
 
 	file := &database.File{
-		ID:   fileID,
-		Path: types.FilePath(path),
-		// Store source directory for restore path stripping
+		ID:         fileID,
+		Path:       types.FilePath(path),
 		SourcePath: types.SourcePath(s.currentSourcePath),
 		MTime:      info.ModTime(),
 		Size:       info.Size(),