feat: Hermes Zulip native platform plugin — deprecates old standalone service

Builds on the NousResearch Hermes Agent plugin system:

- plugins/platforms/zulip/ — full BasePlatformAdapter implementation
  - Zulip event queue polling (like pi-zulip-extension)
  - DM-first + @mention + @all-bots routing
  - Placeholder→edit streaming UX
  - Typing indicators
  - Deduplication and echo-loop prevention
  - Auto-registers via register(ctx) — zero core changes
  - plugin.yaml with full env var schema
- hermes-zulip-plugin/DEPRECATED.md — migration guide from old subprocess-based service

Configuration:
  Env vars: ZULIP_SITE, ZULIP_EMAIL, ZULIP_API_KEY (required)
  Config: platforms.zulip.extra in Hermes config.yaml

plugins/platforms/zulip/__init__.py

@@ -0,0 +1,3 @@
+from .adapter import register
+
+__all__ = ["register"]