Add custom backend configuration

docs/internal/adapters.md

@@ -38,6 +38,12 @@ resolves its own defaults and definition-required inputs.
 with `promptkit.WithArtifactReader`, passes the engine through the HTTP
 adapter's consumer-owned `Runner` interface, and starts the server.
 
+All three CLI paths assemble the engine from the same resolved prompt, profile,
+and schema directories plus configured custom backends. Each backend is mapped
+to Promptkit's public `Backend` value and registered during engine construction,
+so one constructed server engine retains one immutable backend registry and its
+associated capacity state.
+
 ### HTTP
 
 The handler enforces transport limits and strict JSON decoding before mapping