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/protocol/golden_test.go

@@ -74,6 +74,17 @@ func TestDecodeGoldenServerFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			name:      "session.workspace_bound carries the cwd",
+			json:      `{"type":"session.workspace_bound","sessionId":"s1","workspaceRoot":"/home/kami/Programs/correx","sequence":4,"sessionSequence":1}`,
+			wantType:  TypeWorkspaceBound,
+			eventBear: true,
+			check: func(t *testing.T, m ServerMessage) {
+				if m.WorkspaceRoot != "/home/kami/Programs/correx" {
+					t.Fatalf("workspaceRoot not decoded: %+v", m)
+				}
+			},
+		},
 		{
 			name:      "snapshot_complete (non-event control frame)",
 			json:      `{"type":"snapshot_complete"}`,