Name the setting and value when a size does not parse (closes #429)

A size setting such as chunk_size: banana failed with "invalid size
format: strconv.ParseFloat: parsing "": invalid syntax", naming neither
the setting nor the value. The size decoder now quotes the value, and
Parse checks every size setting after ${...} references are filled in,
so the error reads chunk_size: "banana" is not a size; use a number of
bytes or a unit such as 10MB. This covers chunk_size, blob_size_limit
and s3.part_size, and config set.

Model: opus-5-5

internal/config/config.go

@@ -287,7 +287,8 @@ func Load(path string) (*Config, error) {
 }
 
 // Parse reads YAML config data as Load does, without validating it. It
-// refuses a key that no setting has, applies default values for optional
+// refuses a key that no setting has and a size setting whose value is not a
+// size, naming the setting. It applies default values for optional
 // fields, fills in ${...} references using smartconfig, and expands ~ in
 // paths.
 func Parse(data []byte) (*Config, error) {
@@ -312,8 +313,9 @@ func Parse(data []byte) (*Config, error) {
 		return nil, fmt.Errorf("failed to load config file: %w", err)
 	}
 
-	// A ${...} reference can bring in keys that are not in the file.
-	err = checkFilledKeys(sc.Data())
+	// A ${...} reference can bring in keys that are not in the file, and
+	// is the value of a size setting only once it is filled in.
+	err = checkFilledConfig(sc.Data())
 	if err != nil {
 		return nil, err
 	}