chore: upgrade toolchain to AGP 9.2.1 / Gradle 9 / nixpkgs 26.05

- nixpkgs 25.11 → 26.05; gradle_9 + jdk21; build-tools 35 → 36
- Drop custom gradle wrapper — GRADLE_OPTS works post-nixpkgs PR #449037
- AGP 8.7.3 → 9.2.1; Kotlin plugin removed (built-in since AGP 9)
- compileSdk/targetSdk 34 → 35; buildToolsVersion pinned to 36.0.0
- Java/Kotlin target 17 → 21; resValues feature flag required by AGP 9
- applicationVariants (removed in AGP 9) → androidComponents.onVariants
- Test task: testWeatherReleaseUnitTest → :app:test (all flavors, debug)
- CI artifact path fixed to cover all flavor APKs
- Docs: drop nix develop --command prefix (direnv activates shell)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -6,6 +6,7 @@ Runs on the living-room Sony Bravia KD-65XE9305 (Android 8.0, API 26;
 Android System WebView updates independently via Play Store — currently v138).
 Launched remotely through the Bravia REST IP-control API by Control4
 (driver lives in a separate project). See README.md for the API sequence.
+Build toolchain: AGP 9.2.1, Gradle 9 (nixpkgs 26.05), build-tools 36, compileSdk 35, JDK 21.
 
 ## Lab project — with deviations
 
@@ -20,21 +21,18 @@ This is a lab project (`lab` skill conventions apply) EXCEPT:
 ## NixOS aapt2 workaround
 
 AGP cannot run the Maven-downloaded aapt2 on NixOS (dynamically linked
-generic Linux binary). The flake's dev shell ships a wrapped `gradle` that
-always passes `-Dorg.gradle.project.android.aapt2FromMavenOverride=<sdk
-aapt2>` on the command line; Gradle turns `org.gradle.project.*` system
-props into project properties, which is what AGP reads. It must be a CLI
-arg: the nixpkgs-25.11 gradle wrapper ignores `GRADLE_OPTS` (fixed
-post-25.11, nixpkgs PR #449037 — the flake wrapper can be dropped once
-the pin moves past 25.11). No file rewriting; `gradle.properties` stays
-untouched.
+generic Linux binary). The dev shell sets `GRADLE_OPTS` to pass
+`-Dorg.gradle.project.android.aapt2FromMavenOverride=<sdk aapt2>`;
+Gradle turns `org.gradle.project.*` system props into project properties,
+which is what AGP reads. Works since nixpkgs 26.05 (PR #449037 merged
+post-25.11). No file rewriting; `gradle.properties` stays untouched.
 
-## Commands (user-run, from repo root)
+## Commands (user-run, from repo root, direnv activates devshell)
 
-- Tests:        `nix develop --command gradle --no-daemon :app:testWeatherReleaseUnitTest`
-- All APKs:     `nix develop --command gradle --no-daemon :app:assembleRelease`
+- Tests:        `gradle --no-daemon :app:test`
+- All APKs:     `gradle --no-daemon :app:assembleRelease`
   → `app/build/outputs/apk/<flavor>/release/<flavor>-release.apk`
-- Single APK:   `nix develop --command gradle --no-daemon :app:assembleWeatherRelease`
+- Single APK:   `gradle --no-daemon :app:assembleWeatherRelease`
 - Sideload:     `adb connect <tv-ip>:5555 && adb install -r <apk>`
   (bump `versionCode` in app/build.gradle.kts first for upgrades)
 - Fresh install: `adb uninstall cz.c3c.webviewkiosk && adb install <apk>`