fix(tui): restore Codex harness scrolling - #252

The change looks sound. The new is_running guard correctly prevents synthesized cursor-key or mouse-report bytes from being written into a pty whose child has exited, falling through to scroll_history instead. The Codex-specific path is properly gated on both HarnessProvider::Codex and bracketed_paste == Some(true), so it cannot fire for other providers or for a Codex session whose input layer is not yet up. The residual race between the is_running check and the write is handled by the existing let _ = self.sessions.write(...) discard, and is acknowledged in the updated comment. No bugs introduced. The code index for this repository is cold, so this review saw the diff alone.

No findings.