Update SKILL.md with sub-dependency resolution guidance

skills/kotlin-tooling-agp9-migration/SKILL.md

@@ -136,6 +136,23 @@ Summary:
         }
     }
     ```
+11. **Resolve Sub-dependency Variants (Product Flavors / Build Types)**:
+    Because the new KMP Android library plugin enforces a single-variant architecture, it does not natively understand how to resolve dependencies that publish multiple variants (like `debug`/`release` build types, or product flavors like `free`/`paid`). Configure fallback behaviors using `localDependencySelection`:
+    ```kotlin
+    kotlin {
+        android {
+            localDependencySelection {
+                // Determine which build type to consume from Android library dependencies, in order of preference
+                selectBuildTypeFrom.set(listOf("debug", "release"))
+
+                // If the dependency has a 'tier' dimension, select the 'free' flavor
+                productFlavorDimension("tier") {
+                    selectFrom.set(listOf("free"))
+                }
+            }
+        }
+    }
+    ```
 
 ## Path B: Android App + Shared Module Split