Fix channel directory event-loop blocking

[sdgdllc]

Summary

• offload channel-directory session scanning and atomic cache writes from the gateway event loop
• reuse one in-memory session map for Discord, Slack, enum fallback, and plugin fallback directory entries
• add regression coverage proving slow session scans and slow directory writes do not stop the asyncio ticker

Verification

• C:\Users\m_t_c\.hermes-sdgd\venv\Scripts\python.exe -m pytest tests\gateway\test_channel_directory.py -q --timeout-method=thread
• C:\Users\m_t_c\.hermes-sdgd\venv\Scripts\python.exe -m ruff check gateway\channel_directory.py tests\gateway\test_channel_directory.py

SDGD handoff: SundayGundayLLC/sdgd#880