feat(tui-go): in-session right panel cycles events / changes / off (d)

Press d (or the "panel" palette command) in-session to cycle the right panel:
- events  — the live event stream (default, unchanged)
- changes — a token-usage line (tokens + turns, summed from the transcript's
            TurnMetrics) over a git-status-style list of files the session has
            written, each with summed +/− from its diff. ^x still opens the full
            diff.
- off     — hide the panel; the output transcript takes the full width.

changesRows derives everything from data already in the model (router-turn
metrics + the tool entries' unified diffs), so no new protocol. Footer + ? help
updated; "changes" preview kind added.

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

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

@@ -261,7 +261,7 @@ func (m Model) renderFooter() string {
 		} else {
 			// Trimmed to fit ~100 cols: tools/stats/model are all reachable via `p cmds`,
 			// so the footer keeps the high-traffic keys + the new transcript nav/copy.
-			hints = []string{hint("i", "message"), hint("^↑↓", "msgs"), hint("y", "copy"), hint("e", "events"), hint("t", "tools"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
+			hints = []string{hint("i", "message"), hint("^↑↓", "msgs"), hint("y", "copy"), hint("e", "events"), hint("d", "panel"), hint("l", "back"), hint("p", "cmds"), hint("q", "quit")}
 		}
 		if m.copiedFlash != 0 && m.frame-m.copiedFlash < copiedFlashFrames {
 			hints = append(hints, lipgloss.NewStyle().Foreground(t.P.OK).Background(bg).Bold(true).Render("✓ copied"))
@@ -295,12 +295,68 @@ func (m Model) renderMain(h int) string {
 	leftW := m.width * 63 / 100
 	rightW := m.width - leftW
 
-	// In-session / approval: output transcript over the event stream.
+	// In-session / approval. The right panel cycles (d): events, changes (token usage +
+	// changed files), or off (output takes the full width).
+	if m.rightPanel == 2 {
+		return m.theme.box("output", m.routerRows(m.width-4, h-2), m.width, h, true)
+	}
 	left := m.theme.box("output", m.routerRows(leftW-4, h-2), leftW, h, true)
-	right := m.theme.box("events", m.eventRows(rightW-4, h), rightW, h, false)
+	var right string
+	if m.rightPanel == 1 {
+		right = m.theme.box("changes", m.changesRows(rightW-4, h), rightW, h, false)
+	} else {
+		right = m.theme.box("events", m.eventRows(rightW-4, h), rightW, h, false)
+	}
 	return lipgloss.JoinHorizontal(lipgloss.Top, left, right)
 }
 
+// changesRows is the in-session "changes" panel: a token-usage line plus a git-status-style
+// list of files the session has written (summed +/- from each write's diff). ^x opens the
+// full diff. Drives the right panel when rightPanel == 1.
+func (m Model) changesRows(w, h int) []string {
+	t := m.theme
+	turns, toks := 0, 0
+	for _, e := range m.routerMessages[m.selectedID] {
+		if e.Role == "router" && e.Metrics != nil {
+			turns++
+			toks += e.Metrics.TotalTokens
+		}
+	}
+	rows := []string{
+		t.span("tokens ", t.P.Faint) + t.span(itoa(toks), t.P.FgStrong) +
+			t.span("  · turns ", t.P.Faint) + t.span(itoa(turns), t.P.Dim),
+		"",
+	}
+
+	type chg struct{ add, del int }
+	var order []string
+	seen := map[string]*chg{}
+	for _, e := range m.routerMessages[m.selectedID] {
+		if e.Role != "tool" || !isUnifiedDiff(e.Content) {
+			continue
+		}
+		p, a, d := diffSummary(e.Content)
+		if seen[p] == nil {
+			seen[p] = &chg{}
+			order = append(order, p)
+		}
+		seen[p].add += a
+		seen[p].del += d
+	}
+	if len(order) == 0 {
+		return append(rows, t.span("no file changes yet", t.P.Faint))
+	}
+	rows = append(rows, t.span("changes  (^x for full diff)", t.P.Faint))
+	for _, p := range order {
+		c := seen[p]
+		icon := lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.Bg).Render("✎ ")
+		delta := lipgloss.NewStyle().Foreground(t.P.OK).Background(t.P.Bg).Render("+"+itoa(c.add)) +
+			t.span(" ", t.P.Bg) + lipgloss.NewStyle().Foreground(t.P.Bad).Background(t.P.Bg).Render("−"+itoa(c.del))
+		rows = append(rows, icon+t.span(truncate(p, w-16), t.P.Fg)+"  "+delta)
+	}
+	return rows
+}
+
 // renderLauncher is the idle screen: a compact, opencode-style input centered in the main
 // area, with a hideable right rail of status + quick keys. The session list isn't shown —
 // it's a keypress away (R). The input's lower-right shows <workflow> · <model>, Tab-cycled.