feat(tui-go): session list / resume view (E)

R opens a navigable overlay of recent sessions (GET /sessions: short id, status,
workflow/stage, relative age); enter resumes via POST /sessions/{id}/resume, which
replays onto the existing global /stream (no WS re-dial). Fetch/resume errors surface
in the overlay, never panic. stdlib-only (ws.Client.HTTPBase derives the base URL).

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

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

@@ -61,6 +61,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.statsModal())
 	case OverlayIdeas:
 		return m.center(m.ideasModal())
+	case OverlaySessions:
+		return m.center(m.sessionsModal())
 	}
 	return base
 }