Add HTTP and OpenAI integration documentation and rewrite Narratio contract

docs/cli.md

@@ -18,6 +18,11 @@ go run ./cmd/scriptorium render \
 - `scriptorium render`: prepare prompt only; write prepared-run output as `text` or `json`.
 - `scriptorium serve`: start the HTTP server.
 
+Integration references:
+
+- HTTP contract: `docs/integrations/http-api.md`
+- Narratio subprocess contract: `docs/integrations/narratio.md`
+
 ## Common Argument Rules
 
 - `--config` is supported by `run`, `render`, and `serve`.