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>

docs/wiki/The-Dashboard.md

@@ -151,8 +151,9 @@ Worth knowing:
 
 - **Scrollback.** Agent/TUI sessions pull their entire tmux scrollback on first open.
   Shell sessions open from a bounded recent tail so a large transcript cannot stall tab
-  switching; press **Load full history** to pull the rest explicitly. Ordinary Shell scrolling
-  and automatic output recovery stay within the bounded browser buffer.
+  switching. Scrolling to the top of a Shell pane pulls the most recent 1 MiB of its tmux
+  history; press **Load full history** to pull the rest explicitly. Automatic output
+  recovery stays within the bounded browser buffer.
 - **Wheel and touch scrolling** are forwarded into Claude's own transcript on recent Claude
   versions, so the wheel scrolls the conversation rather than the terminal. `Shift+Wheel` is
   always local scrollback. Other CLIs scroll locally.