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/demo.go

@@ -22,6 +22,24 @@ func PreviewFrame(kind string, w, h int) string {
 		m.bgUpdates = 3
 		m.selectedID = "04a546aa"
 
+	case "changes":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.selectedID = "04a546aa"
+		m.sessionEntered = true
+		m.rightPanel = 1
+		if s := m.session("04a546aa"); s != nil {
+			s.CurrentStage = "write_script"
+		}
+		m.routerMessages["04a546aa"] = []RouterEntry{
+			{Role: "user", Content: "add a healthcheck script and document it"},
+			{Role: "router", Content: "I'll write the script.", Metrics: &TurnMetrics{LatencyMs: 1200, TotalTokens: 340}},
+			{Role: "tool", Content: "--- a/healthcheck.sh\n+++ b/healthcheck.sh\n@@ -0,0 +1,4 @@\n+#!/usr/bin/env bash\n+curl -sf localhost:8080/health\n+echo ok\n+exit 0\n"},
+			{Role: "router", Content: "Now the docs.", Metrics: &TurnMetrics{LatencyMs: 820, TotalTokens: 150}},
+			{Role: "tool", Content: "--- a/README.md\n+++ b/README.md\n@@ -1,1 +1,2 @@\n existing line\n+## Health\n"},
+		}
+
 	case "compose":
 		m.connected = true
 		m.currentModel = "llama-cpp:default"