chore(android): install Android SDK + wire AGP 9.2.1 (toolchain proven)

Android SDK installed on this machine (cmdline-tools via brew;
platform-tools + platforms;android-35 + build-tools;35.0.0), local.properties
points Gradle at it (gitignored). Wired AGP 9.2.1 into the version catalog +
google() repos; proved the full chain by building a throwaway android-library
module against SDK 35 (AAR produced), then removed the probe.

Groundwork for AW2+: AGP plugins in the catalog, google() in settings, and the
working recipe in README (incl. the AGP-9-has-built-in-Kotlin gotcha — Android
modules apply ONLY the android plugin, never kotlin.android). Pure JVM modules
still green (218 tests). Framework module stubs stay gated until implemented.

android/README.md

@@ -93,3 +93,30 @@ JUnit Platform (`tasks.test { useJUnitPlatform() }`).
 > Testing target: **≥80% Kover coverage** on the pure modules (`:wire-protocol`,
 > `:session-core`, `:api-client`, `:client-tls` pure half). TDD, immutable data,
 > small focused files — same discipline as the rest of the repo.
+
+## Android SDK setup (proven working)
+
+The pure JVM modules need only a JDK + Gradle. The **Android-framework** modules
+(`:app`, `:terminal-view`, `:host-registry`, `:client-tls-android` — plan AW2+)
+need the Android SDK. This machine is set up and the toolchain is **proven** (an
+AGP library module compiled against SDK 35 and produced an AAR):
+
+- **SDK location:** `/usr/local/share/android-commandlinetools`
+  (installed via `brew install --cask android-commandlinetools`).
+- **Installed packages:** `platform-tools`, `platforms;android-35`, `build-tools;35.0.0`.
+- **`android/local.properties`** (gitignored) points Gradle at it:
+  `sdk.dir=/usr/local/share/android-commandlinetools`.
+- **Shell env** (for `sdkmanager`/`adb`): `export ANDROID_HOME=/usr/local/share/android-commandlinetools`.
+
+### Wiring an Android module (the working recipe)
+
+- Repos: `google()` is in both `pluginManagement` and `dependencyResolutionManagement`
+  in `settings.gradle.kts` (needed to resolve AGP + androidx).
+- Plugin: **AGP 9.2.1** (`libs.plugins.android.library` / `.android.application`),
+  compatible with Gradle 9.6.1.
+- **Gotcha:** AGP 9 has **built-in Kotlin** — apply ONLY the android plugin. Adding
+  `org.jetbrains.kotlin.android` errors with "no longer required since AGP 9.0".
+- Module block: `android { namespace = "…"; compileSdk = 35; defaultConfig { minSdk = 29 } }`.
+
+To add more SDK pieces later (e.g. an emulator image for instrumented tests):
+`sdkmanager "system-images;android-35;google_apis;arm64-v8a" "emulator"`.

android/build.gradle.kts

@@ -5,4 +5,8 @@
 plugins {
     alias(libs.plugins.kotlin.jvm) apply false
     alias(libs.plugins.kotlin.serialization) apply false
+    // AGP 9.2.1 — proven to build against Android SDK 35 with Gradle 9.6.1.
+    // AGP 9 has built-in Kotlin; framework modules apply only these (no kotlin.android).
+    alias(libs.plugins.android.library) apply false
+    alias(libs.plugins.android.application) apply false
 }

android/gradle/libs.versions.toml

@@ -4,6 +4,7 @@
 ## compatibility with the Kotlin Gradle Plugin on Gradle 9.6).
 
 [versions]
+agp = "9.2.1"
 kotlin = "2.3.21"
 kotlinxSerialization = "1.9.0"
 kotlinxCoroutines = "1.10.2"
@@ -35,3 +36,7 @@ unit-test = ["junit-jupiter", "kotlinx-coroutines-test", "turbine", "mockk"]
 [plugins]
 kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
 kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
+android-library = { id = "com.android.library", version.ref = "agp" }
+android-application = { id = "com.android.application", version.ref = "agp" }
+# NOTE: AGP 9+ has BUILT-IN Kotlin support — Android modules apply only the
+# android plugin, NOT a separate org.jetbrains.kotlin.android (it errors out).

android/settings.gradle.kts

@@ -1,8 +1,9 @@
 pluginManagement {
     repositories {
+        google()
         gradlePluginPortal()
         mavenCentral()
-        // google() is only needed by the Android-framework modules (see below).
+        // google() (above) serves AGP for the Android-framework modules.
         // It requires the Android Gradle Plugin / SDK, absent in this env.
     }
 }
@@ -12,6 +13,7 @@ dependencyResolutionManagement {
     // Modules must not declare their own repositories.
     repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
     repositories {
+        google()
         mavenCentral()
     }
     // The version catalog at gradle/libs.versions.toml is auto-registered as `libs`.
@@ -28,11 +30,14 @@ include(":client-tls")
 include(":test-support")
 
 // ── Android-framework modules ───────────────────────────────────────────────────
-// Scaffolded (dirs + build.gradle.kts stubs exist) but NOT included, because this
-// environment has NO Android SDK. Their build scripts apply `com.android.*`
-// plugins that cannot resolve here.
-// TODO(android-sdk): enable when an Android SDK is available.
-// include(":app")
-// include(":terminal-view")
-// include(":host-registry")
-// include(":client-tls-android")
+// The Android SDK IS now installed (see android/README.md → "Android SDK setup"),
+// and AGP 9.2.1 is proven to build against SDK 35 with Gradle 9.6.1. These stay
+// commented out only because their build.gradle.kts are still empty SCAFFOLD STUBS
+// — enable each as its real module is implemented (plan AW2+).
+// Recipe: apply `alias(libs.plugins.android.library)` ONLY (AGP 9 has built-in
+// Kotlin — do NOT add kotlin.android); android { namespace; compileSdk = 35;
+// defaultConfig { minSdk = 29 } }.
+// include(":app")               // AW4 — uses libs.plugins.android.application
+// include(":terminal-view")     // AW3
+// include(":host-registry")     // AW1/AW2 storage half
+// include(":client-tls-android") // AW1 framework half