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/model.go

@@ -278,6 +278,9 @@ type Model struct {
 	diffScrollOffset int
 	modalScroll      int // body scroll for tall fixed-content modals (help, stats)
 	eventStripShown  bool
+	// in-session right panel: 0 = events, 1 = changes (token usage + changed files),
+	// 2 = off (output full-width). Cycled with `d`.
+	rightPanel int
 
 	// artifact viewer (OverlayArtifacts) — populated by the artifact.list response
 	artifacts        []protocol.ArtifactDto