Make thread/channel/message deletion complete (hard delete)

Deleting a thread, channel, or message was a no-op/soft-delete: messages
were only dropped from the plugin outbox (still in hermes' session store,
hence searchable/recoverable) and channels/threads were merely archived.

Now deletion is complete and non-recoverable, with no search trace:

- purge.py (new): hard-delete from hermes' session store (state.db).
  delete_lane wipes a channel's/thread's sessions + messages; deleting a
  messages row also drops it from the FTS5 index via the delete triggers.
  delete_message removes one message, matched by (session, role, exact
  content, closest timestamp) since plugin m_<hex> ids aren't persisted.
- channels.py: delete() hard-deletes the row (and a channel's child
  threads) instead of archiving.
- outbox.py: add delete_lane() (wipe all frames for a lane) and
  message_info() (read a message's final role/text/ts for the match).
- adapter.py: on_channel_delete wipes outbox + session store;
  on_message_delete purges the session-store row per message.
- App: delete confirmations no longer claim history stays for search;
  ChannelStore removes a channel's threads on channel delete.
- Docs updated to describe hard deletion.

app/shared/src/commonMain/kotlin/iris/data/ChannelStore.kt

@@ -29,12 +29,21 @@ class ChannelStore {
     /** Reconcile a server frame into the cache. */
     fun onFrame(frame: Frame) {
         when (frame.type) {
-            TYPE_CHANNEL_CREATED, TYPE_CHANNEL_RENAMED -> upsert(frame)
-            TYPE_CHANNEL_DELETED -> remove(frame)
+            TYPE_CHANNEL_CREATED, TYPE_CHANNEL_RENAMED -> {
+                upsert(frame)
+            }
+
+            TYPE_CHANNEL_DELETED -> {
+                remove(frame)
+            }
+
             TYPE_CHANNEL_LIST -> {
                 frame.payloadAs<ChannelListPayload>()?.let { setAll(it.channels) }
             }
-            else -> Unit
+
+            else -> {
+                Unit
+            }
         }
     }
 
@@ -48,7 +57,10 @@ class ChannelStore {
 
     private fun remove(frame: Frame) {
         val p = frame.payloadAs<ChannelDeletedPayload>() ?: return
-        _channels.value = _channels.value.filter { it.chatId != p.chatId }
+        // A channel delete also removes its threads (child entries); a thread
+        // delete removes just that thread. Both are hard deletes server-side.
+        _channels.value =
+            _channels.value.filter { it.chatId != p.chatId && it.parentChatId != p.chatId }
     }
 
     private fun sorted(list: List<ChannelInfo>): List<ChannelInfo> =
@@ -62,20 +74,16 @@ class ChannelStore {
     // ── UI helpers ────────────────────────────────────────────────────────
 
     /** Channels (default + user channels) for the drawer/rail. */
-    fun channelsForDrawer(): List<ChannelInfo> =
-        _channels.value.filter { it.kind != "thread" }
+    fun channelsForDrawer(): List<ChannelInfo> = _channels.value.filter { it.kind != "thread" }
 
     /** Threads under a channel, for the topic switcher. */
-    fun threadsFor(chatId: String): List<ChannelInfo> =
-        _channels.value.filter { it.kind == "thread" && it.parentChatId == chatId }
+    fun threadsFor(chatId: String): List<ChannelInfo> = _channels.value.filter { it.kind == "thread" && it.parentChatId == chatId }
 
-    fun byId(chatId: String): ChannelInfo? =
-        _channels.value.firstOrNull { it.chatId == chatId }
+    fun byId(chatId: String): ChannelInfo? = _channels.value.firstOrNull { it.chatId == chatId }
 
-    fun defaultChannel(): ChannelInfo? =
-        _channels.value.firstOrNull { it.isDefault } ?: _channels.value.firstOrNull()
+    fun defaultChannel(): ChannelInfo? = _channels.value.firstOrNull { it.isDefault } ?: _channels.value.firstOrNull()
 
     fun clear() {
         _channels.value = emptyList()
     }
-}
+}

app/shared/src/commonMain/kotlin/iris/ui/screens/ChatScreen.kt

@@ -839,7 +839,7 @@ fun ChatScreen(controller: IrisController) {
                     onDismissRequest = { deleteThread = null },
                     title = { Text("Delete topic?") },
                     text = {
-                        Text("Delete \"${t.name}\"? It will be removed from this channel.")
+                        Text("Delete \"${t.name}\"? The topic and all its messages will be permanently deleted.")
                     },
                     confirmButton = {
                         TextButton(onClick = {
@@ -861,9 +861,9 @@ fun ChatScreen(controller: IrisController) {
                     text = {
                         Text(
                             if (n == 1) {
-                                "This message will be deleted for all devices."
+                                "This message will be permanently deleted for all devices and can no longer be found in search."
                             } else {
-                                "These messages will be deleted for all devices."
+                                "These messages will be permanently deleted for all devices and can no longer be found in search."
                             },
                         )
                     },
@@ -1049,7 +1049,7 @@ fun ChatScreen(controller: IrisController) {
         AlertDialog(
             onDismissRequest = { channelToDelete = null },
             title = { Text("Delete channel?") },
-            text = { Text("Delete \"${ch.name}\"? Its history stays available for search.") },
+            text = { Text("Delete \"${ch.name}\"? The channel and all its messages will be permanently deleted.") },
             confirmButton = {
                 TextButton(onClick = {
                     channelToDelete = null