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/server.go

@@ -175,6 +175,10 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		if s := m.session(msg.SessionID); s != nil {
 			s.addEvent(nowMillis(), "ArtifactCreated", msg.ArtifactID)
 		}
+	case protocol.TypeWorkspaceBound:
+		if s := m.session(msg.SessionID); s != nil {
+			s.WorkspaceRoot = msg.WorkspaceRoot
+		}
 	case protocol.TypeApprovalRequired:
 		m.onApprovalRequired(msg)
 	case protocol.TypeApprovalResolved: