skill: add Android skills

.agents/skills/agp-9-upgrade/SKILL.md

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+---
+name: agp-9-upgrade
+description: Upgrades, or migrates, an Android project to use Android Gradle Plugin
+  (AGP) version 9. Do not use this skill for migrating Kotlin Multiplatform (KMP)
+  projects.
+license: Complete terms in LICENSE.txt
+metadata:
+  author: Google LLC
+  last-updated: '2026-06-25'
+  keywords:
+  - Android Gradle Plugin 9
+  - AGP 9
+  - AGP Upgrade
+  - AGP Migration
+  - New AGP DSL
+  - Migrate to built-in Kotlin
+---
+
+## Migration guide
+
+See the [AGP 9 migration guide](references/android/build/releases/agp-9-0-0-release-notes.md) for the major changes, many
+breaking, in AGP 9 compared to AGP 8.
+
+## Requirements
+
+If the user requests to update or migrate to AGP 9, first check the AGP version
+used in the project. If it is lower than 9, stop and ask the user to run the AGP
+Upgrade Assistant in Android Studio to update to the latest stable version of
+AGP, and confirm when done. The user may also request that this requirement be
+skipped; if this is the case, you should update the version of AGP to the latest
+stable version as part of the AGP 9 migration. See the
+[AGP 9 migration guide](references/android/build/releases/agp-9-0-0-release-notes.md) for how to do this.
+
+Each version of AGP has its own set of compatibilities with other tools, such as
+Gradle, JDK, and Kotlin. The release notes for each of these versions will
+include a **Compatibility** table indicating the minimum versions for these
+tools.
+
+Do not use this skill for KMP projects, as they are unsupported.
+
+## Steps
+
+If AGP is already at 9 or higher, then do the following:
+
+### Step 1: Update dependencies
+
+If KSP (`com.google.devtools.ksp`) is used in the project, ensure it is on
+version 2.3.6 or higher.
+
+If Hilt is used in the project, ensure it is on version 2.59.2 or higher.
+
+### Step 2: Migrate to built-in Kotlin
+
+See [the guide](references/android/build/migrate-to-built-in-kotlin.md) for detailed information.
+
+### Step 3. Migrate to the new AGP DSL
+
+See [the guide](references/android/build/releases/agp-9-0-0-release-notes.md) for detailed information.
+
+See also [gradle-recipes](references/recipes.md) for examples on how to migrate old code to code
+that is compatible with AGP 9 and the new DSL.
+
+### Step 4. Migrate kapt to KSP or legacy-kapt
+
+If KSP (`com.google.devtools.ksp`) or kapt (`org.jetbrains.kotlin.kapt`) are
+used in the project, see [KSP, kapt, and legacy-kapt](references/ksp-kapt.md) for detailed migration
+steps.
+
+### Step 5. BuildConfig
+
+If any Android module contains custom BuildConfig fields, see [BuildConfig](references/buildconfig.md)
+for detailed information.
+
+### Step 6. Update gradle.properties
+
+After the migration, check gradle.properties. Remove the following flags:
+
+1. android.builtInKotlin
+2. android.newDsl
+3. android.uniquePackageNames
+4. android.enableAppCompileTimeRClass
+
+Additionally, delete all temporary files you've created.
+
+## Guidelines
+
+- Never write or run python scripts.
+- Only search the Gradle dependency cache when inspecting external dependencies, and only as a last resort.
+- Never add `android.disallowKotlinSourceSets=false` to `gradle.properties`.
+- When verifying changes, don't run the `clean` task. This is a waste of time.
+
+## Verification
+
+After migration, verify the following:
+
+1. Gradle IDE sync succeeds.
+2. `./gradlew help` succeeds.
+3. `./gradlew build --dry-run` succeeds.
+
+## Troubleshooting
+
+Paparazzi v2.0.0-alpha04 and lower versions have issues with AGP 9. See
+[references/paparazzi-gradle-9.md](references/paparazzi-gradle-9.md) for details.