skill: add Android skills

.agents/skills/play-billing-library-version-upgrade/SKILL.md

@@ -0,0 +1,93 @@
+---
+name: play-billing-library-version-upgrade
+description: Use this skill when upgrading or migrating an Android project from any
+  legacy Google Play Billing Library (PBL) version to the latest stable version of
+  PBL.
+license: Complete terms in LICENSE.txt
+metadata:
+  author: Google LLC
+  last-updated: '2026-07-02'
+  keywords:
+  - android
+  - play billing
+  - play billing library
+  - pbl
+  - upgrade
+  - migration
+  - deprecation
+  - google play
+---
+
+## Phase 0: Intent Message
+
+**Reporting Action**: Before proceeding, immediately tell the user: "I will
+upgrade Play Billing Library to the latest version."
+
+## Phase 1: Discovery \& Situational Awareness
+
+1. **Primary Check (Build Version)** : Locate the project's billing dependency (e.g., `com.android.billingclient:billing`) in `build.gradle`, `build.gradle.kts`, or `libs.versions.toml`.
+2. **Initial Compilation Test**: Attempt to sync and build the project immediately.
+3. **Fallback Discovery (Effective Version)** :
+   - **Trigger**: Only if the build fails immediately, scan the source code for deprecated artifacts.
+   - **Logic** : The presence of deprecated APIs indicates the **"Effective
+     Version"** ---defined as the version where those specific APIs were **last
+     available**, not when they were introduced.
+   - **Example** : If `SkuDetails` is present, treat the baseline as **PBL v7** or earlier (regardless of the version string in `build.gradle`).
+4. **Identify Target \& Path** : Access the version tool or release notes to find the latest stable version and calculate a \[Direct/Stepped\] migration path based on the **Effective Version** baseline.
+
+- **Calculate Migration Path** :
+  - If the **Effective Version** is within 2 major versions of the target: Plan a **Direct Migration**.
+  - If it is more than 2 major versions behind: Plan a **Stepped
+    Migration**. Migrate by two major versions at a time (e.g., v4 -\> v6 -\> v8) until you are within two versions of the target.
+- **Reporting Action**: Before proceeding, tell the user: "I've detected you are effectively on PBL \[Current\] and the latest is \[Target\]. I am planning a \[direct/stepped\] migration path."
+
+## Phase 2: Contextual Document Mapping \& Planning
+
+For every major version jump identified in your path, you **MUST** synthesize
+instructions from:
+
+- **[Migration Guide](https://developer.android.com/google/play/billing/migrate-gpblv%5BX%5D)** (where `[X]` is the target major version).
+- **Release Highlights** : The "Deprecations" and "Breaking Changes" sections of the relevant [Release Notes](references/android/google/play/billing/release-notes.md).
+- **Developer Documentation**: Consult your knowledge of the Google Play Billing documentation regarding the relevant features used in this app (e.g., Subscriptions, One-Time Products).
+- **Develop the Plan**: Identify every specific code change required (API removals, class replacements, logic shifts) and print this out as a checklist.
+
+## Phase 3: Instructions for Execution
+
+*Reporting Action: For each of the following steps, give a brief explanation of
+what you will be doing prior to execution, and a brief summary of what you
+accomplished afterwards.*
+
+### Step 1: SDK \& Environment Alignment
+
+- **Action** : Update `build.gradle` to meet SDK requirements (e.g., "PBL 9 requires `compileSdk` 35").
+- **Gradle Version**: Verify if the new library requires a newer Android Gradle Plugin (AGP) or Kotlin version.
+
+### Step 2: Intent-based Refactoring
+
+Analyze the intent of the existing code rather than performing purely textual
+string replacement.
+
+- **Action** : You **MUST** follow all deprecation instructions and refactor patterns from **both** the [references/migration-logic.md](references/migration-logic.md) section, the official migration guides, and the general documentation pages identified in Phase 2.
+- **Verification** : Verify you are doing **all steps from all documentation** and then making sure you follow the specific directions from the checklist in the references.
+
+### Step 3: Sequential Verification (Only applicable for Stepped Migrations)
+
+1. **Upgrade** to the first major intermediate version in your path.
+2. **Run `./gradlew assembleDebug`** to verify no intermediate breaking changes were missed.
+3. **Repeat** until you reach the final target version.
+
+### Step 4: Final Validation Checklist
+
+1. **Smart Checklist Verification**:
+2. Open [references/version-checklist.md](references/version-checklist.md) and locate the **Smart Version-Specific Checklist**.
+3. **Action**: For every version between your \[Detected Effective Version\] and \[Detected New Version\], verify that every item has been addressed in the code using "Find in Files" or structural analysis.
+4. **Tests** : Run all unit and implementation tests (`./gradlew test`).
+5. **Clean Build** : Verify the project completes a full clean build: `./gradlew clean assembleDebug`. Then, run `./gradlew sync` and `./gradlew build` so that the user can immediately test the new version manually.
+
+## Final Report
+
+Explain the "Why" to the developer:
+
+- "I updated your SDK to \[Version\] because PBL \[Version\] requires it for \[Reason from docs\]."
+- "I removed your custom `retryConnection()` logic because it is now handled natively by the library using `enableAutoServiceReconnection()`."
+- "Successfully upgraded from PBL \[Old\] to PBL \[New\] and verified with unit tests. Based on an analysis of features in the latest library and this application's current feature set, I suggest exploring \[New Feature\] (e.g., Prepaid Plans or Installments) from the latest release because it is now available but not yet implemented."