fix(gateway): add missing UTF-8 encoding to file I/O preventing crashes on Windows

On Windows, Python's open() defaults to the system locale encoding
(e.g. cp1254 for Turkish, cp1252 for Western European) instead of
UTF-8. The gateway already uses ensure_ascii=False in json.dumps()
to preserve Unicode characters in chat messages, but the
corresponding open() calls lack encoding="utf-8". This mismatch
causes UnicodeEncodeError / UnicodeDecodeError when users send
non-ASCII messages (Turkish, Japanese, Arabic, emoji, etc.) through
Telegram, Discord, WhatsApp, or Slack on Windows.

The project already fixed this for .env files in hermes_cli/config.py
(line 624) but the gateway module was missed.

Files fixed:
- gateway/session.py: session index + JSONL transcript read/write (5 calls)
- gateway/channel_directory.py: channel directory read/write (3 calls)
- gateway/mirror.py: session index read + transcript append (2 calls)

gateway/mirror.py

@@ -73,7 +73,7 @@ def _find_session_id(platform: str, chat_id: str) -> Optional[str]:
         return None
 
     try:
-        with open(_SESSIONS_INDEX) as f:
+        with open(_SESSIONS_INDEX, encoding="utf-8") as f:
             data = json.load(f)
     except Exception:
         return None
@@ -103,7 +103,7 @@ def _append_to_jsonl(session_id: str, message: dict) -> None:
     """Append a message to the JSONL transcript file."""
     transcript_path = _SESSIONS_DIR / f"{session_id}.jsonl"
     try:
-        with open(transcript_path, "a") as f:
+        with open(transcript_path, "a", encoding="utf-8") as f:
             f.write(json.dumps(message, ensure_ascii=False) + "\n")
     except Exception as e:
         logger.debug("Mirror JSONL write failed: %s", e)