Add HeXi bot codebase: custom plugins, web frontends, tests

- hexi core: message handling, rate limiting, cooldown, plugin manager
- Custom plugins: BF stats, daily check-in, quotes, persona cards, etc.
- Community plugins vendored under hexi/plugins with local fixes
- Web admin frontends (learning-chat, persona-admin), unified hexi/web
- Tests for rate_limit/cooldown/memes/persona; poetry.lock

Co-Authored-By: Claude <noreply@anthropic.com>

plugin_template/nonebot_plugin_template/web/__init__.py

@@ -0,0 +1,3 @@
+"""web 子应用：插件自带管理后台(FastAPI)，由 hub 自动挂载到 /api/<id>。"""
+
+from .admin import build_admin_app  # noqa: F401