Remove external music generation handoff

docs/ARCHITECTURE.md

@@ -29,8 +29,7 @@ macOS App
 │   ├── Project Versioning
 │   └── Validation
 ├── Integrations
-│   ├── OpenAI
-│   └── Suno Field-Fill Integration
+│   └── OpenAI
 └── Persistence
     └── Local project/version storage
 ```
@@ -82,17 +81,10 @@ and explicitly apply the processed text before any Suno handoff.
     and providers can be changed later.
 -   Do not log secrets or full authorization headers.
 
-## Suno Boundary
+## External Generation Boundary
 
-The initial requirement is an app-managed web surface for the Suno
-Create page in Custom mode. This avoids relying on a particular user
-browser. The integration attempts to fill the supported title,
-lyrics and style fields but does not press Generate. If a field cannot
-be filled, it returns a field-specific result so Presentation can report
-it and offer the approved text for copying. Treat this integration as
-replaceable because website UI/behavior can change. The field filler is
-an adapter behind a protocol so it can be replaced without changing
-SongProject, prompt compilation or UI review logic.
+The application prepares final lyrics and style prompts but does not
+open, embed or control an external music-generation service.
 
 ## Instrument Selector Boundary