Add optional SSH execution backend

[Xiangyue-Zhang]

Summary

• add an optional local/ssh execution backend for tool execution and experiment monitoring
• keep controller state local while allowing code, logs, PID checks, and GPU queries to run on one remote host
• update README, AI_GUIDE, architecture docs, and slash-command guidance for the new execution mode

Validation

• python3 -m unittest discover -s tests -p "test*.py"
• python3 -m compileall core tests