Summary

• defer destructive slash commands like /new, /clear, /reset, and /undo when the CLI is in the middle of an active turn
• interrupt the current turn without turning the slash command itself into queued interrupt text, then run the destructive command only after chat() fully unwinds
• add regression coverage for the new deferred-command path and keep the existing destructive-confirmation tests green

Fixes #24701.

Testing

• uv sync --frozen --extra all
• uv run --frozen pytest tests/cli/test_destructive_slash_confirm.py tests/cli/test_cli_new_session.py tests/cli/test_prompt_text_input_thread_safety.py
• uv run --frozen ruff check .
• uv run --frozen python scripts/check-windows-footguns.py --all
• git diff --check

Attribution

• local lint-diff replay still surfaces repo-wide ty noise, but it did not report new diagnostics on this diff's changed cli.py ranges or the new regression test file; treated as preexisting_unrelated