refactor(amethyst): drop PlayAmethyst — appfunctions doesn't need it

The previous wiring forced Amethyst to be `open`, added a 30-line
PlayAmethyst subclass that only implemented AppFunctionConfiguration
.Provider, and used tools:replace="android:name" in the play manifest
to swap classes. The justification was that the appfunctions runtime
discovers @AppFunction host classes via Application.appFunctionConfiguration.

Reading the KSP-generated dispatcher
($AmethystAppFunctions_AppFunctionInvoker.kt) shows that's only half
true. The invoker passes a default-construction fallback lambda when
instantiating the host class, and ConfigurableAppFunctionFactory takes
that fallback as a constructor argument. Provider is only consulted to
*override* construction — required for classes with non-default
constructors, optional otherwise.

AmethystAppFunctions has a no-arg constructor, so:
  * PlayAmethyst is deleted entirely
  * Amethyst goes back to `class Amethyst : Application()` (no `open`)
  * Play manifest reverts to plain `android:name=".Amethyst"`, no
    tools:replace gymnastics

Verified by assemblePlayDebug (APK builds clean) and the merged play
manifest still pinning the appfunctions service. If we ever add a
host class with constructor parameters (an Account-injected one, say),
we'll need to add Provider back — kdoc on AmethystAppFunctions
documents that.

amethyst/src/main/java/com/vitorpamplona/amethyst/Amethyst.kt

@@ -26,7 +26,7 @@ import com.vitorpamplona.amethyst.service.nests.AppForegroundRecycleHook
 import com.vitorpamplona.quartz.utils.Log
 import com.vitorpamplona.quartz.utils.LogLevel
 
-open class Amethyst : Application() {
+class Amethyst : Application() {
     init {
         Log.minLevel = if (BuildConfig.DEBUG) LogLevel.DEBUG else LogLevel.ERROR
         Log.d("AmethystApp") { "Creating App $this" }

amethyst/src/play/AndroidManifest.xml

@@ -2,13 +2,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
         xmlns:tools="http://schemas.android.com/tools">
 
-    <!-- Play flavor overrides android:name with PlayAmethyst, which is
-         the same Amethyst Application class plus the
-         AppFunctionConfiguration.Provider surface required by
-         androidx.appfunctions. F-Droid keeps the plain Amethyst. -->
     <application
-        android:name=".PlayAmethyst"
-        tools:replace="android:name">
+        android:name=".Amethyst">
 
         <service
             android:name=".service.notifications.PushNotificationReceiverService"

amethyst/src/play/java/com/vitorpamplona/amethyst/PlayAmethyst.kt

@@ -1,52 +0,0 @@
-/*
- * Copyright (c) 2025 Vitor Pamplona
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy of
- * this software and associated documentation files (the "Software"), to deal in
- * the Software without restriction, including without limitation the rights to use,
- * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
- * Software, and to permit persons to whom the Software is furnished to do so,
- * subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
- * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
- * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
- * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
- * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- */
-package com.vitorpamplona.amethyst
-
-import androidx.appfunctions.service.AppFunctionConfiguration
-import com.vitorpamplona.amethyst.appfunctions.AmethystAppFunctions
-
-/**
- * Play-flavor Application subclass that adds the
- * [AppFunctionConfiguration.Provider] surface required by
- * androidx.appfunctions.
- *
- * Lives in the `play` source set only because [AppFunctionConfiguration]
- * (and the entire appfunctions library) is a play-channel `playImplementation`
- * dependency — the F-Droid build does not ship Google AI libraries and
- * continues to use the unmodified [Amethyst] Application class.
- *
- * Registered via `tools:replace="android:name"` in
- * `amethyst/src/play/AndroidManifest.xml`.
- */
-class PlayAmethyst :
-    Amethyst(),
-    AppFunctionConfiguration.Provider {
-    override val appFunctionConfiguration: AppFunctionConfiguration
-        // Lazy single instance — AmethystAppFunctions is stateless (it
-        // reaches into Amethyst.instance on every call), but the runtime
-        // is free to invoke functions concurrently, so we hand back the
-        // same object for every dispatch rather than rebuilding.
-        get() =
-            AppFunctionConfiguration
-                .Builder()
-                .addEnclosingClassFactory(AmethystAppFunctions::class.java) { AmethystAppFunctions() }
-                .build()
-}

amethyst/src/play/java/com/vitorpamplona/amethyst/appfunctions/AmethystAppFunctions.kt

@@ -46,8 +46,14 @@ import kotlinx.coroutines.withTimeoutOrNull
  * ships without any Google AI dependencies.
  *
  * Plain class, no inheritance — the KSP compiler discovers `@AppFunction`
- * methods and generates the dispatcher glue. Construction is wired in
- * [PlayAmethyst.appFunctionConfiguration].
+ * methods and generates the dispatcher glue (see
+ * `amethyst/build/generated/ksp/playDebug/.../$AmethystAppFunctions_AppFunctionInvoker.kt`).
+ * The generated invoker constructs this class via its default no-arg
+ * constructor, so no `AppFunctionConfiguration.Provider` is required on
+ * the Application. If we ever add an @AppFunction host class with
+ * constructor parameters, we'll need to register a factory via Provider —
+ * the docs nudge that direction, but the runtime does not require it for
+ * default-constructed classes.
  *
  * Only read-only verbs are exposed so far. Write verbs (post, follow, zap)
  * are intentionally deferred until we resolve the signer-prompt flow for