Refuse config keys that no setting has (closes #419)

config.Parse walks the file as written and fails on the first key that
matches no setting, naming its dotted path and line, so a typo in
exclude or prune_grace_period no longer falls back silently. Keys that a
${...} reference brings in are checked once smartconfig has filled it
in, and named without a line. YAML merge keys and a top-level env
section, which smartconfig reads, still load; an anchor that contains
itself is an error, and each anchored map is checked once per type.
config set refuses such a key before writing, with the same error, and
sets a key under env like any other value. The key-to-field lookup
moves from internal/cli to config.SettingType so both use it.

Model: opus-5-5

internal/config/config.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"reflect"
 	"sort"
 	"strings"
 	"time"
@@ -286,15 +287,37 @@ func Load(path string) (*Config, error) {
 }
 
 // Parse reads YAML config data as Load does, without validating it. It
-// applies default values for optional fields, fills in ${...} references
-// using smartconfig, and expands ~ in paths.
+// refuses a key that no setting has, applies default values for optional
+// fields, fills in ${...} references using smartconfig, and expands ~ in
+// paths.
 func Parse(data []byte) (*Config, error) {
+	// smartconfig drops line numbers, so keys are checked in the data as
+	// written.
+	var root yaml.Node
+
+	err := yaml.Unmarshal(data, &root)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse config: %w", err)
+	}
+
+	err = checkKeys(&root, reflect.TypeFor[Config](), "",
+		map[*yaml.Node]bool{}, map[nodeAndType]bool{})
+	if err != nil {
+		return nil, err
+	}
+
 	// Load config using smartconfig for interpolation
 	sc, err := smartconfig.NewFromReader(bytes.NewReader(data))
 	if err != nil {
 		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())
+	if err != nil {
+		return nil, err
+	}
+
 	cfg := &Config{
 		// Set defaults
 		BlobSizeLimit:    defaultBlobSizeLimit,