Add engine-scoped backend registration

engine.go

@@ -14,6 +14,7 @@ import (
 	artifactadapter "gitea.maximumdirect.net/eric/promptkit/internal/artifact"
 	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
 	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
 	"gitea.maximumdirect.net/eric/promptkit/internal/profile"
 	"gitea.maximumdirect.net/eric/promptkit/internal/profile/builtin"
@@ -108,8 +109,10 @@ type Config struct {
 // NewEngine applies options in argument order and ignores nil options. Within
 // each prompt-source, profile-source, in-memory-profile, schema-source,
 // model-client, and artifact-reader category, the last non-nil valid option
-// replaces earlier options in that category. An invalid option fails
-// construction even if a later option would replace it.
+// replaces earlier options in that category. WithBackend is the additive
+// exception: unique registrations accumulate, and a repeated backend ID is an
+// error rather than a replacement. An invalid option fails construction even
+// if a later option would replace it.
 type Option interface {
 	apply(*engineOptions) error
 }
@@ -126,6 +129,7 @@ type engineOptions struct {
 	promptDefs      promptdef.Repository
 	profiles        profile.Repository
 	memoryProfiles  profile.Repository
+	backends        []domain.Backend
 	validator       validate.Validator
 	promptSource    bool
 	profileSource   bool
@@ -303,8 +307,9 @@ func WithSchemaFile(path string) Option {
 //
 // Options are applied in order according to [Option]. PromptDir is required
 // unless a prompt-source option is present. Construction validates option
-// arguments and in-memory profiles but defers reading and validating prompt,
-// file-backed profile, and schema contents until Prepare or Run needs them.
+// arguments, in-memory profiles, and backend registrations but defers reading
+// and validating prompt, file-backed profile, and schema contents until Prepare
+// or Run needs them.
 //
 // NewEngine returns an error matching ErrInvalidConfig for invalid
 // configuration or options. It does not perform model requests or require
@@ -336,7 +341,7 @@ func NewEngine(cfg Config, opts ...Option) (*Engine, error) {
 		profiles = profile.NewOverlayRepository(options.memoryProfiles, profiles)
 	}
 
-	backendRegistry, err := backend.NewRegistry(nil)
+	backendRegistry, err := backend.NewRegistry(options.backends)
 	if err != nil {
 		return nil, fmt.Errorf("%w: failed to construct backend registry: %v", ErrInvalidConfig, err)
 	}