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/overlays.go

@@ -830,6 +830,7 @@ func (m Model) helpBody() []string {
 	})
 	section("session", []kb{
 		{"i", "compose a message"},
+		{"d", "right panel: events / changes / off"},
 		{"s", "toggle chat / steering mode"},
 		{"w", "show workflows (idle screen)"},
 		{"y", "copy the selected message (OSC52)"},