Add engine-scoped backend registration

docs/consumers/pkg-promptkit.md

@@ -97,6 +97,32 @@ For programmatic profiles,
 [`OpenAICompatibleProfile`](../../profiles.go) converts ordinary
 OpenAI-compatible settings into a value accepted by `WithProfiles`.
 
+### Register A Custom Backend
+
+Register a reusable OpenAI-compatible connection once, then select it from a
+profile:
+
+```go
+engine, err := promptkit.NewEngine(promptkit.Config{
+	PromptDir: "prompts",
+},
+	promptkit.WithBackend(promptkit.Backend{
+		ID:        "local",
+		Endpoint:  "http://localhost:8000/v1",
+		APIKeyEnv: "LOCAL_LLM_API_KEY",
+	}),
+	promptkit.WithProfiles(promptkit.Profile{
+		ID:        "local-summary",
+		BackendID: "local",
+		Model:     "example-model",
+	}),
+)
+```
+
+Registrations belong to one engine and custom IDs cannot replace built-ins.
+The [`Backend` and `WithBackend` GoDoc](../../backends.go) defines validation,
+copying, uniqueness, and request-default behavior.
+
 ## Credentials
 
 File-backed profiles name an environment variable; in-memory profiles can

docs/roadmap/implementation.md

@@ -369,6 +369,8 @@ Do not duplicate every profile-parser validation case at the engine boundary.
 
 ## Stage 3 — Consumer Registration
 
+**Status:** Complete.
+
 ### Goal
 
 Expose the engine-scoped extension point for additional unique