Document local PromptKit backend support

docs/integrations/pkg-promptkit.md

@@ -1,11 +1,11 @@
 # PromptKit Integration
 
 Notarius pins
-[`gitea.maximumdirect.net/eric/promptkit` v0.2.0](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.2.0)
+[`gitea.maximumdirect.net/eric/promptkit` v0.3.0](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.3.0)
 as its in-process prompt engine. The upstream
-[Go package consumer guide](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.2.0/docs/consumers/pkg-promptkit.md)
+[Go package consumer guide](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.3.0/docs/consumers/pkg-promptkit.md)
 owns the public engine API, and the upstream
-[format reference](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.2.0/docs/formats.md)
+[format reference](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.3.0/docs/formats.md)
 owns prompt, profile, and schema file contracts.
 
 ## Supported Boundary
@@ -18,10 +18,16 @@ Notarius relies on the root `promptkit` package to:
   a direct session ID, prompt identity, and profile selection;
 - return rendered debug material, validated structured output, selected
   profile, backend, effective model metadata, and token usage;
+- register the optional conventional `local` backend through `BackendLocal`,
+  `LocalBackend`, and `WithBackend`;
 - distinguish structured-output validation failure from execution failure; and
 - identify a missing explicit profile through `ErrProfileNotFound` and backend
   admission exhaustion through `ErrCapacityExceeded`.
 
+The pinned
+[`BackendLocal`, `LocalBackend`, and `WithBackend` API](https://gitea.maximumdirect.net/eric/promptkit/src/tag/v0.3.0/backends.go)
+owns the registration and backend-capacity contract.
+
 Notarius does not use PromptKit's optional `ArtifactReader`. It materializes
 source and reference content itself and supplies owned inline artifacts at the
 adapter boundary. It also retains responsibility for pipeline retries,
@@ -38,8 +44,9 @@ Notarius records PromptKit's selected backend ID and effective reasoning
 setting as optional run-manifest provenance. Endpoint-only profiles have no
 backend ID. Debug prompt material also retains the selected backend ID and
 PromptKit's stable lower-case `effective_model_params` JSON, which may include
-`backend_id`. Notarius production configuration does not expose user-defined
-PromptKit backend registration.
+`backend_id`. Notarius production configuration exposes one optional
+conventional `local` registration. It does not expose a general user-defined
+PromptKit backend registry. Endpoint-only profiles remain supported unchanged.
 
 Notarius retains its application-wide scheduled client around the PromptKit
 adapter. PromptKit may apply a narrower limit for the selected backend;
@@ -55,7 +62,8 @@ module assets, maps its transport-neutral completion contract, prepares and
 executes requests, validates output, records provenance, captures debug
 material, redacts errors, and preserves timeout ownership.
 [Configuration](../config.md#promptkit-profiles) defines how a Notarius
-configuration selects one PromptKit profile source.
+configuration selects one PromptKit profile source and optionally registers
+the conventional local backend.
 
 PromptKit API or format changes outside this boundary are not implicitly
 supported. Updating the pinned version requires reviewing the adapter and