Document public library usage

docs/internal/adapters.md

@@ -8,6 +8,7 @@ This document describes implemented adapter/repository boundaries and their curr
 
 - `internal/adapter/cli`: CLI command parsing, app wiring, stdout/stderr handling, exit codes.
 - `internal/adapter/http`: HTTP request/response mapping for `POST /v1/runs`.
+- root package `scriptorium`: public Go library facade for preparing and running prompt requests.
 - `internal/promptdef`: filesystem prompt-definition repository.
 - `internal/profile`: filesystem execution-profile repository.
 - `internal/artifact`: input artifact reader.
@@ -30,6 +31,13 @@ HTTP adapter:
 - Output: JSON success/error body with mapped status codes.
 - Success metadata includes token usage plus cache usage counters.
 
+Public library facade:
+
+- Input: typed `scriptorium.RunRequest` values.
+- Output: typed `PreparedRun` and `RunResult` values plus public sentinel errors.
+- Custom LLM behavior is injected with `WithLLMClient`; otherwise the default OpenAI-compatible client is used.
+- Public types are facade types converted at the package boundary; internal domain types remain internal.
+
 Filesystem repositories:
 
 - Input: prompt/profile YAML files under configured directories.