fix: emit SSE keepalive comments while buffering cleanup tags

When the SSE filter suppresses text deltas (buffering inside a
memory_cleanup/yuyay-response tag), no bytes reached the client,
causing opencode's SSE read timeout to fire. Now emits ':keepalive'
SSE comments during suppression to keep the connection alive.

src/mnemosyne/gateway.py

@@ -273,13 +273,16 @@ class SSECleanupFilter:
             cleaned = self._feed(text)
 
             if cleaned is None or not cleaned:
-                # Suppressed — also remove the preceding "event:" line
+                # Suppressed — remove the preceding "event:" line
                 # to avoid malformed SSE (event header with no data)
                 if out_lines and out_lines[-1].startswith(b"event:"):
                     out_lines.pop()
-                # Also remove trailing empty line if present
                 if out_lines and out_lines[-1] == b"":
                     out_lines.pop()
+                # Emit an SSE comment to keep the connection alive.
+                # Without this, clients time out during long tag buffering.
+                out_lines.append(b": keepalive")
+                out_lines.append(b"")
                 continue
             elif cleaned == text:
                 # No change — pass through original bytes