fix(panel): never auto-close panels on child exit

[peters]

Summary

• A clean child exit (exit code 0) still auto-removed the panel: exit in a shell, Ctrl-D / /exit in an agent, or any successful one-shot command would make the panel vanish.
• PR fix(panel): keep panels open when child process fails or exits non-zero #180 already kept failure exits open; this finishes the job for the success case so the panel is never closed automatically. The user always closes panels manually via the X or the "Close" context menu item.
• Drops the now-unused machinery: Panel::should_close_after_exit, the should_close_for_exit_status helper, Board::exited_panels, and the lifecycle loop that fed exited ids into panels_to_close.
• SSH "Reconnect" still works: SSH panels were already exempted from auto-close, and child_exited() (used to flip SshConnectionStatus::Disconnected) is untouched.

Test plan

• Open a Shell panel, type exit, confirm the panel stays open and can be closed via the X.
• Open a Claude panel, run /exit (or Ctrl-C then exit), confirm the panel stays open.
• Open a Command panel that runs a short-lived script (e.g. echo hi), confirm the panel stays open after the command finishes.
• Open an SSH panel, terminate the connection (exit), confirm the panel stays open and Reconnect still works.
• Click the X / Close menu item on each, confirm manual close works as before.