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.

gateway-plugin/channels.py

@@ -293,20 +293,28 @@ class ChannelDirectory:
         return self.get(chat_id)
 
     def delete(self, chat_id: str) -> dict[str, Any] | None:
-        """Soft-delete (archive) a channel. History stays for search.
+        """Hard-delete a channel or thread (and, for a channel, its threads).
 
-        The default channel cannot be deleted. Returns the (archived) entry,
-        or ``None`` when the id is unknown / is the default.
+        The row is removed from the directory entirely -- not recoverable. The
+        caller (adapter) is responsible for wiping the lane's history from the
+        outbox and the hermes session store so no search trace survives.
+
+        The default channel cannot be deleted. Returns the (deleted) entry, or
+        ``None`` when the id is unknown / is the default.
         """
         with self._lock:
             row = self._conn.execute(
-                "SELECT is_default FROM channels WHERE chat_id = ?", (chat_id,)
+                "SELECT * FROM channels WHERE chat_id = ?", (chat_id,)
             ).fetchone()
             if row is None or row["is_default"]:
                 return None
-            self._conn.execute("UPDATE channels SET archived = 1 WHERE chat_id = ?", (chat_id,))
+            entry = _row_to_entry(row)
+            self._conn.execute("DELETE FROM channels WHERE chat_id = ?", (chat_id,))
+            # A channel takes its threads with it.
+            if entry["kind"] != KIND_THREAD:
+                self._conn.execute("DELETE FROM channels WHERE parent_chat_id = ?", (chat_id,))
             self._conn.commit()
-        return self.get(chat_id)
+        return entry
 
     # ── reads ─────────────────────────────────────────────────────────────