revert(cli): drop scrollback box width clamp (#25975), restore full-width borders (#26163)

#25975 (salvaging #24403) clamped decorative scrollback Panels and
streaming box rules to `max(32, min(width, 56))` as a defense against
terminal-emulator reflow when columns shrink. On any modern wide
terminal this made the response/reasoning borders look stubby — 56
cols inside a 200-col viewport.

#26137 (salvaging #25981, by @OutThisLife) landed a more fundamental
fix: prompt_toolkit's `_output_screen_diff` is monkey-patched so its
reserve-vertical-space cursor move no longer pushes chrome into
scrollback at all. With that in place, the clamp is no longer
load-bearing for the chrome-into-scrollback class of bugs — the
remaining risk is purely cosmetic reflow of *already stamped*
Panel borders during an aggressive column shrink, which we now
accept as a tradeoff for restoring proper full-width rendering.

Changes:
- `_scrollback_box_width()` returns `max(32, width)` (just the floor,
  no upper cap). All 10 call sites stay valid.
- Updated `test_scrollback_box_width_caps_to_resize_safe_value` to
  the new `test_scrollback_box_width_returns_viewport_width` asserting
  full-width passthrough above the 32-col floor.

Floor of 32 is kept so `'─' * (w - 2)` math stays positive on tiny
terminals.

Refs #18449 #19280 #22976 (the original reflow class) and #25975
(the clamp this reverts).

tests/cli/test_cli_status_bar.py

@@ -349,20 +349,27 @@ class TestCLIStatusBar:
         assert cli_obj._tui_input_rule_height("top", width=90) == 1
         assert cli_obj._tui_input_rule_height("bottom", width=90) == 1
 
-    def test_scrollback_box_width_caps_to_resize_safe_value(self):
-        """Decorative scrollback boxes clamp to a width small enough that
-        moderate terminal shrinks don't cause reflow into scrollback."""
+    def test_scrollback_box_width_returns_viewport_width(self):
+        """Decorative scrollback boxes use the full viewport width.
+
+        The previous clamp (max 56 cols) was reverted in favour of the
+        prompt_toolkit ``_output_screen_diff`` monkey-patch landed in
+        #26137, which keeps chrome out of scrollback at the source.
+        We accept that an aggressive column-shrink may visually reflow
+        already printed Panel borders — that's a cosmetic artifact of
+        stamped scrollback history, not a live-render bug.
+        """
         from cli import HermesCLI
 
-        # Floor at 32 — narrow terminals still get something usable.
+        # Floor at 32 — narrow terminals still get something usable
+        # (avoids negative ``'─' * (w - 2)`` math).
         assert HermesCLI._scrollback_box_width(20) == 32
         assert HermesCLI._scrollback_box_width(32) == 32
-        # Cap at 56 — wide terminals don't get full-width boxes.
-        assert HermesCLI._scrollback_box_width(80) == 56
-        assert HermesCLI._scrollback_box_width(120) == 56
-        assert HermesCLI._scrollback_box_width(200) == 56
-        # Mid-range passes through up to the cap.
+        # Above the floor, return the actual viewport width — no cap.
         assert HermesCLI._scrollback_box_width(48) == 48
+        assert HermesCLI._scrollback_box_width(80) == 80
+        assert HermesCLI._scrollback_box_width(120) == 120
+        assert HermesCLI._scrollback_box_width(200) == 200
 
     def test_agent_spacer_reclaimed_on_narrow_terminals(self):
         cli_obj = _make_cli()