Document the bundled OpenRouter configuration

engine.go

@@ -101,8 +101,9 @@ type Engine struct {
 }
 
 // ConfigurationSources selects ordered ConfigKit underlays and an optional
-// highest-priority main input. Each underlay's inputs strictly compose within
-// their group; later underlays overlay earlier ones. A nil Main uses
+// highest-priority main input. By default a bundled OpenRouter underlay precedes
+// the caller underlays; [WithBundledOpenRouter] can disable it. Inputs within
+// each underlay strictly compose; later underlays overlay earlier ones. A nil Main uses
 // ConfigKit's empty source. A non-nil Main is passed through unchanged and
 // may fail construction if invalid.
 type ConfigurationSources struct {
@@ -222,7 +223,9 @@ func WithSchemaFile(path string) Option {
 // NewEngine constructs an Engine from configuration and options.
 //
 // ConfigKit composes caller-selected underlays in their given order, followed
-// by the optional main input. Construction validates the final effective
+// by the optional main input, after the default-on bundled OpenRouter underlay.
+// The bundle adds a root backend and profiles to inventories, but no prompts
+// or implicit profile selection. Construction validates the final effective
 // backend, profile, and prompt definitions and captures referenced prompt
 // content. A missing prompt collection is allowed. Schema documents and
 // request artifacts remain runtime resources.