fix(cli): warn when FileStores delete() fails

Delete() returning false on a still-existing file meant we silently lost
scratch state for MLS group / key-package / message stores. Route the
four call sites through a `deleteOrWarn` helper that logs via the
KMP-safe quartz Log when the file refuses to disappear.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/stores/FileStores.kt

@@ -23,8 +23,15 @@ package com.vitorpamplona.amethyst.cli.stores
 import com.vitorpamplona.quartz.marmot.mip00KeyPackages.KeyPackageBundleStore
 import com.vitorpamplona.quartz.marmot.mls.group.MarmotMessageStore
 import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupStateStore
+import com.vitorpamplona.quartz.utils.Log
 import java.io.File
 
+private fun File.deleteOrWarn(tag: String) {
+    if (!delete() && exists()) {
+        Log.w(tag) { "Failed to delete $absolutePath" }
+    }
+}
+
 /**
  * Test-harness file stores. **Unencrypted** — the production interfaces
  * document that implementations MUST encrypt at rest, but this CLI is a
@@ -53,8 +60,8 @@ class FileMlsGroupStateStore(
     override suspend fun load(nostrGroupId: String): ByteArray? = stateFile(nostrGroupId).takeIf { it.exists() }?.readBytes()
 
     override suspend fun delete(nostrGroupId: String) {
-        stateFile(nostrGroupId).delete()
-        retainedFile(nostrGroupId).delete()
+        stateFile(nostrGroupId).deleteOrWarn("FileMlsGroupStateStore")
+        retainedFile(nostrGroupId).deleteOrWarn("FileMlsGroupStateStore")
     }
 
     override suspend fun listGroups(): List<String> =
@@ -112,7 +119,7 @@ class FileKeyPackageBundleStore(
     override suspend fun load(): ByteArray? = file.takeIf { it.exists() }?.readBytes()
 
     override suspend fun delete() {
-        file.delete()
+        file.deleteOrWarn("FileKeyPackageBundleStore")
     }
 }
 
@@ -137,6 +144,6 @@ class FileMarmotMessageStore(
     override suspend fun loadMessages(nostrGroupId: String): List<String> = file(nostrGroupId).takeIf { it.exists() }?.readLines()?.filter { it.isNotBlank() } ?: emptyList()
 
     override suspend fun delete(nostrGroupId: String) {
-        file(nostrGroupId).delete()
+        file(nostrGroupId).deleteOrWarn("FileMarmotMessageStore")
     }
 }