fix: remove orphaned SSE event headers when suppressing text deltas

When the cleanup filter suppresses a text delta (buffering inside a
tag), the preceding 'event: content_block_delta' header was left in
the output, producing malformed SSE that caused opencode to retry
rapidly and freeze. Now removes the event header alongside the data
line.

src/mnemosyne/gateway.py

@@ -136,12 +136,17 @@ class SSECleanupFilter:
         """Process a raw SSE chunk (may contain multiple lines).
 
         Returns the (possibly rewritten) chunk to forward downstream.
+        When a text delta is suppressed (buffering inside a tag), the
+        preceding ``event:`` header line is also removed to avoid
+        producing malformed SSE (event header with no data).
         """
         lines = raw_chunk.split(b"\n")
         out_lines: list[bytes] = []
 
         for line in lines:
             if not line.startswith(b"data: "):
+                # Buffer event: lines — only emit them if the next data: line
+                # is kept.  Non-event lines (empty lines, comments) pass through.
                 out_lines.append(line)
                 continue
 
@@ -173,15 +178,18 @@ class SSECleanupFilter:
             # Feed text into buffer and get cleaned output
             cleaned = self._feed(text)
 
-            if cleaned is None:
+            if cleaned is None or not cleaned:
-                # Entire chunk is being buffered (inside a tag) — suppress
+                # Suppressed — also 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()
                 continue
             elif cleaned == text:
                 # No change — pass through original bytes
                 out_lines.append(line)
-            elif not cleaned:
-                # Text was entirely a cleanup tag — suppress this event
-                continue
             else:
                 # Rewrite the delta with cleaned text
                 delta["text"] = cleaned