fix(remote): load a remote session's scrollback from the remote tmux

A remote SSH session's local pane runs `ssh -t … tmux new-session -A` (or
`attach`), so the local tmux only sees the remote tmux client's full-screen
repaints and keeps a fraction of the history: measured 48 of 279 lines while
output streamed, none after a reattach. The remote pane, where the shell
runs, keeps all of it.

A full-history load (`GET /api/sessions/:id/terminal?full=1`) of a remote
session now reads the scrollback rows from the remote pane in one ssh round
trip (`#{history_size}` + `capture-pane -S -<N> -E -1`, exact `=name:`
target, newest bytes kept by a remote `tail -c`), and the local capture
reads only its visible frame (`-S 0`), spliced below them
(`PaneCaptureOptions.scrollbackOverride`). The local frame and caret are
kept because the live stream keeps drawing into the local pane.

Fail-soft and bounded: connection options from buildSshConnectionArgs(),
the shared remote-ssh limiter (a slot waited for longer than 1.5 s is given
up), a 5 s timeout, and a 30 s per-host back-off after any failure, which
keeps the local capture as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/remote-sessions.md

@@ -251,6 +251,18 @@ unreachable host answers "unknown", which also means do not revive. The answer
 is cached per session and cleared whenever the pane is next seen alive, so a
 stale `true` from one transport drop can never revive the NEXT clean exit.
 
+## Terminal history from the remote tmux
+
+The local pane runs the ssh attach, so it only sees the remote tmux client's
+full-screen repaints and keeps a fraction of the scrollback (none after a
+reattach). A full-history load (`GET /api/sessions/:id/terminal?full=1`) of a
+remote session therefore reads the scrollback rows from the REMOTE pane in one
+ssh round trip (`src/remote-pane-history.ts`) and splices them above the local
+visible frame and caret. Bounded (5 s, the shared ssh limiter) and fail-soft:
+any failure keeps the local capture and skips that host for 30 s. How much
+history exists is the remote tmux's own `history-limit`, which the launch
+command does not set (tmux's default is 2000 lines).
+
 ## File access over SSH
 
 A remote case's `workingDir` is an absolute path on the **remote** host