fix(ssh): stop reconnecting after the remote shell exits on purpose

[kipavy]

Fixes #180.

The bug

With Settings → Hosts → Persistent sessions off, typing exit in an SSH terminal disconnected and then immediately reconnected to the same host. With persistent sessions on, exit behaved correctly.

Root cause

ssh-closed-<id> carried no reason for the close, so handleSessionClosed treated a deliberate exit exactly like a dropped link and started the reconnect backoff. Persistent sessions escaped it only by accident: their attach-only probe fails with SESSION_ENDED, which tears the tab down.

A deliberate exit and a dropped link are distinguishable on the wire. Verified against a local sshd:

• exit from an interactive shell → client_input_channel_req: channel 0 rtype exit-status
• transport killed mid-session → zero exit-status messages

The fix

• channel_io.rs records an ExitStatus / ExitSignal seen before Eof/Close and ships that flag as the ssh-closed-<id> payload.
• handleSessionClosed ends the session (disconnect + drop the tab, via closeSession) instead of reconnecting when the flag is set — matching what persistent sessions already do when their session ends on the host.
• Persistent sessions keep the old path: their tmux/screen wrapper also exits on a detach, so the attach probe stays the judge there.
• The three terminal views (MainPanel, PaneTerminal, MobileSessionView) each built the same deps object; they now share one sessionClosed helper.

Tests

• channel_io::tests::only_a_reported_exit_counts_as_a_remote_exit — exit-status/exit-signal count, Eof/Close/other messages do not.
• reconnectBackoff.test.ts — clean exit ends the session; clean exit on a persistent session still reconnects; a drop with no exit-status still reconnects.
• tsc --noEmit clean.

Live run

Headless container on this branch, persistent sessions toggled off, real SSH host (ssh-host-1):

• Typed exit → the connected session left the store within 1 s and stayed gone for the 13 s watched; its tab disappeared from the tab bar. No reconnect.
• Control, same build: kill -9 on the remote sshd-session pids (a drop, no exit-status) → session went connecting at 2 s and back to connected at 4 s. Reconnect still works.