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. ${...}
references and YAML merge keys still load. config set refuses such a key
before writing. 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"
@@ -286,9 +287,24 @@ 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](), "")
+	if err != nil {
+		return nil, err
+	}
+
 	// Load config using smartconfig for interpolation
 	sc, err := smartconfig.NewFromReader(bytes.NewReader(data))
 	if err != nil {