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"`.