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.

docs/04-wire-protocol.md

@@ -376,6 +376,11 @@ Answer an interactive picker.
 {"type":"channel.set_default","id":16,"chat_id":"android:chan_7","payload":{}}
 ```
 
+`channel.delete` is a **hard delete**: the channel/thread row is removed from
+the directory and the lane's history is wiped from the outbox and the hermes
+session store (no search trace, not recoverable). Deleting a channel also
+removes its threads. The default channel cannot be deleted.
+
 ### `search`
 
 ```json
@@ -409,10 +414,12 @@ Load a page of messages for a chat/thread (initial open, scroll-up pagination).
 
 ### `message.delete`
 
-Delete the given message(s) from a chat/thread. The server removes them from the
-outbox (so `history`/`sync` no longer return them) and broadcasts
-`message.deleted` to every device. Idempotent: a message already gone (pruned by
-retention) still yields a `message.deleted` broadcast so live caches drop it.
+Completely delete the given message(s) from a chat/thread. The server removes
+them from the outbox (so `history`/`sync` no longer return them) **and** from
+the hermes session store (so no search trace survives and they are not
+recoverable), then broadcasts `message.deleted` to every device. Idempotent: a
+message already gone (pruned by retention) still yields a `message.deleted`
+broadcast so live caches drop it.
 
 ```json
 {"type":"message.delete","id":30,"chat_id":"android:default","thread_id":null,