Ensure kgpy repl exits on .x(0)

[briangu]

Summary

• propagate SystemExit from the Klong event loop back to the console task and record the resulting exit code
• stop the REPL task without raising so the main loop can shut down cleanly and exit with the recorded status
• add a regression test that runs kgpy -d with .x(0) on stdin to make sure the interactive REPL terminates

Testing

• python -m unittest tests.test_sys_fn.TestSysFn.test_eval_sys_exit -v
• python -m unittest

---

https://chatgpt.com/codex/tasks/task_e_6871a2c441408332b5d7be7f2f658baf