feat(tui-go): page-scroll the diff/preview/command fullscreen viewer

The ^x fullscreen viewer (OverlayDiff) backs every tool preview — split diff
for write/edit, the command card for shell, plain lines otherwise, plus any
transcript tool-output — but only scrolled one line per keypress, making a
long diff tedious to read.

- Add paging to the viewer, matching the OUTPUT free-scroll contract: ↑↓/jk
  by line, PgUp/PgDn by a full body, ^u/^d by half, g/G to the ends. New
  scrollDiff() clamps to [0, diffMaxScroll] so paging past either edge lands
  exactly (no dead presses). diff_scroll_test.go (2): clamp + no-op-when-fits.
- Update the modal hint to advertise paging (line / page / ends / close) and
  add a "diff / preview viewer (^x)" section to the ? help overlay. Also note
  the T3+ confirm-again behaviour in the approval-band help.

Approval ergonomics re-audit (BACKLOG §E): the spec'd band still holds —
single-key y/n/e for T0-T2, mandatory second-key confirm for T3+ (HighTier),
navigable queue with an i/n indicator, docked band never modal-stacked; all
covered green by approval_test.go. The scroll granularity above was the only
gap, now closed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/overlays.go

@@ -353,6 +353,20 @@ func (m Model) diffMaxScroll() int {
 	return max
 }
 
+// scrollDiff moves the diff/preview/command modal offset by delta rows, clamped to
+// [0, diffMaxScroll] so paging past either end lands exactly on the edge (no dead
+// presses unwinding an overshoot — same contract as the OUTPUT free-scroll).
+func (m *Model) scrollDiff(delta int) {
+	off := m.diffScrollOffset + delta
+	if mx := m.diffMaxScroll(); off > mx {
+		off = mx
+	}
+	if off < 0 {
+		off = 0
+	}
+	m.diffScrollOffset = off
+}
+
 func (m Model) diffModal() string {
 	if cmd := m.pendingCommand(); cmd != nil {
 		return m.commandModal(cmd)
@@ -390,7 +404,7 @@ func (m Model) diffModal() string {
 	for _, ln := range lines {
 		b.WriteString(ln + "\n")
 	}
-	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "scroll"}, {"^x/esc", "close"}}))
+	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "line"}, {"PgUp/Dn", "page"}, {"g/G", "ends"}, {"^x/esc", "close"}}))
 
 	modal := t.Overlay.Width(w).Render(b.String())
 	return m.center(modal)
@@ -424,7 +438,7 @@ func (m Model) commandModal(a *Approval) string {
 	for i := off; i < end; i++ {
 		b.WriteString(lines[i] + "\n")
 	}
-	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "scroll"}, {"^x/esc", "close"}}))
+	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "line"}, {"PgUp/Dn", "page"}, {"g/G", "ends"}, {"^x/esc", "close"}}))
 	return m.center(t.Overlay.Width(w).Render(b.String()))
 }
 
@@ -742,11 +756,18 @@ func (m Model) helpModal() string {
 		{"G / I", "grants / idea board"},
 	})
 	section(&b, "approval band", []kb{
-		{"y / a / enter", "approve once"},
+		{"y / a / enter", "approve once (T3+ asks again to confirm)"},
 		{"n / r", "reject"},
 		{"e / s", "steer (add a note)"},
 		{"A", "approve-always — choose scope"},
 		{"↑ / ↓", "walk the pending queue"},
+		{"^x", "fullscreen the diff / command preview"},
+	})
+	section(&b, "diff / preview viewer (^x)", []kb{
+		{"↑ / ↓", "scroll a line"},
+		{"PgUp / PgDn", "scroll a page (^u / ^d half)"},
+		{"g / G", "jump to top / end"},
+		{"^x / esc", "close"},
 	})
 	b.WriteString(modalHints(t, [][2]string{{"any key", "close"}}))
 	modal := t.Overlay.Width(w).Render(b.String())