skill: add Android skills

.agents/skills/agp-9-upgrade/references/buildconfig.md

@@ -0,0 +1,54 @@
+When an Android module contains custom BuildConfig fields, the following steps
+are necessary to ensure a correct build.
+
+### Step 1: Enable the buildConfig build feature
+
+In a build script:
+
+    android {
+      buildFeatures {
+        buildConfig = true
+      }
+    }
+
+In custom build-logic for an app module:
+
+    extensions.configure<com.android.build.api.dsl.ApplicationExtension> {
+      buildFeatures {
+        buildConfig = true
+      }
+    }
+
+In custom build-logic for a library module:
+
+    extensions.configure<com.android.build.api.dsl.LibraryExtension> {
+      buildFeatures {
+        buildConfig = true
+      }
+    }
+
+In custom build-logic using `CommonExtension`:
+
+    extensions.configure<com.android.build.api.dsl.CommonExtension> {
+      buildFeatures {
+        buildConfig = true
+      }
+    }
+
+### Step 2: Migrate to the new API
+
+Use the **addCustomBuildConfigFields** recipe from the [gradle-recipes](https://developer.android.com/agents/skills/build/agp/agp-9-upgrade/references/recipes)
+repository.
+
+**IMPORTANT:** For `BuildConfigField`s with a type of `String`, the `value` field
+*must* include quotation marks as part of the String. For example:
+
+    BuildConfigField(
+      type = "String",
+      value = "\"Some value\"",
+      comment = "Optional comment",
+    )
+
+It is an **error** if the `value` field doesn't include quotation marks as
+part of the String. For example, `value = "Some value"` **is an error** . This is
+because the `value` is written out literally.