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>

src/web/routes/session-routes.ts

@@ -163,6 +163,7 @@ import {
   toAttachedSessionRemote,
   toSessionRemote,
 } from '../../remote-hosts.js';
+import { fetchRemoteScrollback } from '../../remote-pane-history.js';
 import {
   checkDockerAvailable,
   checkDockerConfigDrift,
@@ -3116,6 +3117,19 @@ export function registerSessionRoutes(
           maxCaptureBytes: terminalBufferMaxBytes,
         }
       : {};
+    // A remote session's local pane only sees the remote tmux client's repaints,
+    // so its own scrollback is a fraction of the real one: read that from the
+    // remote tmux and splice it above the local frame. Null (host unreachable,
+    // no remote session, back-off) keeps the local capture as before.
+    if (isFullReload && session.remote && muxName && captureOpts.historyLimitLines !== undefined) {
+      const remoteScrollback = await fetchRemoteScrollback(
+        session.remote,
+        session.id,
+        captureOpts.historyLimitLines,
+        terminalBufferMaxBytes
+      );
+      if (remoteScrollback !== null) captureOpts.scrollbackOverride = remoteScrollback;
+    }
     const liveMuxBuffer =
       muxName && typeof ctx.mux.captureActivePaneBuffer === 'function'
         ? ctx.mux.captureActivePaneBuffer(muxName, captureOpts)