Complete the public facade adapter boundary

docs/consumers/pkg-scriptorium.md

@@ -113,11 +113,16 @@ counts. `RenderedPrompt`, `RenderedMessage`, `CacheControl`, and
 
 `ArtifactReader` implements
 `Read(context.Context, ArtifactRef) (*Artifact, error)`. Supplying it through
-`WithArtifactReader` replaces the engine's default inline and file reader for
-all inputs. Reader errors remain available through `errors.Is` alongside
-`ErrArtifactLoad`; a nil artifact with no error is treated as an artifact-load
-failure. Readers must supply artifact metadata and should not retain or mutate
-the caller's values.
+`WithArtifactReader` replaces, rather than extends, the engine's default inline
+and file reader for every input. Omitting the option retains that default;
+`WithArtifactReader(nil)` makes engine construction fail with
+`ErrInvalidConfig`.
+
+Reader failures are surfaced as errors matching `ErrArtifactLoad` while
+preserving the reader's original error identity for `errors.Is`. A `(nil, nil)`
+reader response is also an artifact-load failure. Readers are responsible for
+artifact metadata, although the engine assigns the input-map name when the
+returned name is empty; readers should not retain or mutate caller values.
 
 ## Requests, Inputs, And Overrides
 
@@ -191,5 +196,8 @@ Public methods preserve these sentinel checks through `errors.Is`:
 - `ErrLLMGenerate`
 - `ErrValidation`
 
-For interface selection and operational responsibilities, see the
-[consumer integration overview](api.md).
+`ErrProfileRequired` and `ErrAPIKeyEnvMissing` each also match
+`ErrInvalidRequest`, so callers can select either the broad request category or
+the specific condition.
+
+For the HTTP interface, see the [HTTP API reference](../api.md).