Enable explicit configuration imports and overlays

internal/config/load.go

@@ -21,8 +21,10 @@ type LoadOptions struct {
 // Load resolves one ConfigKit document and retains its inspection snapshot.
 func Load(options LoadOptions) (*configkit.Result[Document], error) {
 	var source configkit.Source
+	fileBacked := false
 	if options.Explicit {
 		source = configkit.File(options.Path)
+		fileBacked = true
 	} else {
 		selected := ""
 		for _, candidate := range defaultConfigSearchPaths {
@@ -41,11 +43,14 @@ func Load(options LoadOptions) (*configkit.Result[Document], error) {
 			source = configkit.NoSource()
 		} else {
 			source = configkit.File(selected)
+			fileBacked = true
 		}
 	}
 
 	result, err := configkit.Load(source, configkit.Options[Document]{
 		Definition: documentDefinition(),
+		Imports:    fileBacked,
+		Overlay:    options.Overlay,
 		Overrides:  options.Overrides,
 	})
 	if err != nil {

internal/config/load_test.go

@@ -174,3 +174,56 @@ func TestLoadDiscoveryErrorQuotesPath(t *testing.T) {
 		t.Fatalf("discovery error did not quote path safely: %v", err)
 	}
 }
+
+func TestLoadComposesExplicitImportsAndSelectedOverlay(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(dir, "fragments"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.MkdirAll(filepath.Join(dir, "overlays"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	base := filepath.Join(dir, "config.yml")
+	writeApplicationFile(t, base, "configkit:\n  imports:\n    - glob: fragments/*.yml\nprompt_dir: ./base-prompts\nserver:\n  addr: :8081\n")
+	writeApplicationFile(t, filepath.Join(dir, "fragments", "a.yml"), "backends:\n  alpha:\n    endpoint: http://alpha.invalid/v1\n")
+	writeApplicationFile(t, filepath.Join(dir, "fragments", "b.yml"), "backends:\n  beta:\n    endpoint: http://beta.invalid/v1\n")
+	writeApplicationFile(t, filepath.Join(dir, "overlays", "local.yml"), "prompt_dir: ./overlay-prompts\nserver:\n  addr: :9090\n")
+
+	result, err := Load(LoadOptions{Path: base, Explicit: true})
+	if err != nil || result.Config.PromptDir != "base-prompts" || len(result.Config.Backends) != 2 || len(result.Sources()) != 3 {
+		t.Fatalf("disjoint imports did not compose: result=%+v err=%v", result, err)
+	}
+	result, err = Load(LoadOptions{Path: base, Explicit: true, Overlay: "local", Overrides: []configkit.Override{{
+		Path: configkit.Keys("server", "addr"), Value: ":7070", Flag: "--addr",
+	}}})
+	if err != nil || result.Config.PromptDir != "overlay-prompts" || result.Config.Server.Addr != ":7070" || len(result.Sources()) != 5 {
+		t.Fatalf("overlay or explicit override changed: result=%+v err=%v", result, err)
+	}
+	if _, err := Load(LoadOptions{Path: base, Explicit: true, Overlay: "missing"}); err == nil {
+		t.Fatal("missing selected overlay accepted")
+	}
+	writeApplicationFile(t, filepath.Join(dir, "fragments", "b.yml"), "backends:\n  alpha:\n    endpoint: http://conflict.invalid/v1\n")
+	if _, err := Load(LoadOptions{Path: base, Explicit: true}); err == nil {
+		t.Fatal("conflicting import accepted")
+	} else {
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("conflict lost structured diagnostic: %v", err)
+		}
+	}
+}
+
+func TestLoadRejectsOverlayWithoutBase(t *testing.T) {
+	useConfigSearchPaths(t, filepath.Join(t.TempDir(), "absent.yml"))
+	if _, err := Load(LoadOptions{Overlay: "local"}); err == nil {
+		t.Fatal("overlay without base accepted")
+	} else {
+		var loadError *configkit.LoadError
+		if !errors.As(err, &loadError) {
+			t.Fatalf("no-base overlay lost structured diagnostic: %v", err)
+		}
+	}
+	if _, err := Load(LoadOptions{Overlay: "default"}); err != nil {
+		t.Fatalf("default must select no overlay: %v", err)
+	}
+}