feat(tui): show session workspace (cwd) in the status bar

SessionWorkspaceBoundEvent already recorded the bound workspace root but
it was never surfaced. Map it to a new session.workspace_bound frame
(sealed ServerMessage variant, auto-registered; re-emitted on reconnect
via the snapshot replay), carry it on Session.WorkspaceRoot, and show it
home-abbreviated in the status bar (⌂ ~/Programs/correx). Golden test
pins the decode.

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

@@ -73,6 +73,7 @@ func PreviewFrame(kind string, w, h int) string {
 		m.sessionEntered = true
 		if s := m.session("04a546aa"); s != nil {
 			s.CurrentStage = "write_script"
+			s.WorkspaceRoot = "/home/kami/Programs/correx"
 			s.Events = sampleEvents()
 			s.Pending = &Approval{
 				RequestID: "req-1", SessionID: "04a546aa", Tier: "T3", Risk: "MEDIUM",