fix(terminal): let a Shell pane's scroll-up reach tmux history

A burst of output leaves a Shell pane with about one screen of browser
scrollback, because tmux repaints the burst instead of scrolling it,
while tmux itself keeps every line. Shell declined the scroll-to-top
re-pull other modes use, and the Load full history button renders only
once a replay was truncated, so a Shell tab under 1 MiB could not
scroll back at all.

The scroll gesture now pulls ?full=1&tail=TERMINAL_TAIL_SIZE, the same
bound a tab switch loads; the route's existing tail cut marks longer
histories 'tail', so the banner still offers the unbounded pull. A
window no longer than the browser's buffer is not rewritten.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/terminal-ui.js

@@ -3304,9 +3304,9 @@ Object.assign(CodemanApp.prototype, {
   /**
    * Post-scroll companion to _noteTerminalUserScroll: hitting the TOP of the
    * buffer while scrolling up gives the app a chance to pull the rest of tmux's
-   * scrollback (issue #205, see _maybeRefetchFullHistory). Shell sessions decline
-   * automatic pulls because their captures can be large; their banner button is
-   * the explicit path. Must be called AFTER scrollLines(), since the check is on
+   * scrollback (issue #205, see _maybeRefetchFullHistory). Shell sessions pull a
+   * bounded window because their captures can be large; their banner button is
+   * the unbounded path. Must be called AFTER scrollLines(), since the check is on
    * the resulting position, and it is deliberately not folded into
    * _noteTerminalUserScroll for exactly that reason.
    */