Add public artifact reader support

docs/consumers/pkg-scriptorium.md

@@ -24,7 +24,8 @@ fields:
 | `HTTPClient` | Optional HTTP client for that built-in client. It is cloned; a positive `Timeout` on it is the transport cap and takes precedence over `Config.Timeout`. A non-positive client timeout is treated as unset. |
 
 Nil options are ignored. Invalid construction, including
-`WithLLMClient(nil)`, returns an error matching `ErrInvalidConfig`.
+`WithLLMClient(nil)` and `WithArtifactReader(nil)`, returns an error matching
+`ErrInvalidConfig`.
 
 Profile and request `timeout_seconds` values select a per-generation-call
 deadline independently of the transport cap. An explicit request override of
@@ -38,7 +39,8 @@ Source options replace their matching directory source:
 - profiles: `WithProfileFS(fsys, root)`, `WithProfileFile(path)`, and
   `WithProfiles(profiles...)`;
 - schemas: `WithSchemaFS(fsys, root)`, `WithSchemaFile(path)`; and
-- LLM client: `WithLLMClient(client)`.
+- LLM client: `WithLLMClient(client)`; and
+- artifact reader: `WithArtifactReader(reader)`.
 
 `fs.FS` prompt-content and schema paths stay inside their configured roots.
 Single-file prompt and profile sources are selected by their YAML `id`, not
@@ -109,6 +111,14 @@ The exported constants define these serialized values:
 counts. `RenderedPrompt`, `RenderedMessage`, `CacheControl`, and
 `StructuredOutputSpec` are the public shapes used by injected LLM clients.
 
+`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.
+
 ## Requests, Inputs, And Overrides
 
 `RunRequest` fields are `PromptID`, `PromptVersion`, `ProfileID`,
@@ -172,8 +182,10 @@ Public methods preserve these sentinel checks through `errors.Is`:
 - `ErrInvalidRequest`
 - `ErrPromptNotFound`
 - `ErrProfileNotFound`
+- `ErrProfileRequired`
 - `ErrPromptLoad`
 - `ErrProfileLoad`
+- `ErrAPIKeyEnvMissing`
 - `ErrArtifactLoad`
 - `ErrPromptRender`
 - `ErrLLMGenerate`