refactor(xxm): consolidate 4 bot implementations into unified xmpp_agent_core.py

- Merge bot_base.py, gateway/scripts/xmpp_bot.py, bots/*, xmpp_bot_rest.py
  into single xmpp_agent_core.py with --agent flag (xxm|mohe|zhiwei|xiaoguo)
- Add xxm_bot.py wrapper (encoding=utf-8 for Windows exec)
- Fix slixmpp connect() API: use host=/port= keyword args (was tuple)
- Clean up orphans: bots/, scripts/, hermes_state.py, xmpp_bot.py, xmpp_bot_rest.py
- Add docs/CLEANUP_PLAN.md documenting the migration
- Update README.md project structure
- Also: fix WeChat agent path resolution (relative paths)

xmpp_zhiwei_bot.py

@@ -2,4 +2,4 @@
 """Wrapper for xmpp_agent_core.py --agent zhiwei"""
 import sys, os
 sys.argv = [sys.argv[0], '--agent', 'zhiwei']
-exec(open(os.path.join(os.path.dirname(__file__), 'xmpp_agent_core.py')).read())
+exec(open(os.path.join(os.path.dirname(__file__), 'xmpp_agent_core.py'), encoding='utf-8').read())