Establish canonical developer documentation structure

docs/internal/sources.md

@@ -147,6 +147,18 @@ Source packages do not persist run state.
 - `internal/usecase/integration_test.go`
 - `engine_test.go`
 
+## Change Recipe
+
+When updating prompt, profile, schema, or built-in-profile assets:
+
+1. Keep files valid for their strict loader and source boundary.
+2. Keep maintained examples and fixtures secret-free.
+3. Run focused prompt, profile, schema, or validation tests for the changed
+   source.
+4. Update the [configuration contract](../config.md) and every affected
+   external contract; update this document when loading or precedence mechanics
+   change.
+
 ## Architectural Invariants
 
 - Prompt/profile identity comes from YAML `id`.