docs(openspec): archive realtime-booking-chat，同步主 specs

- archive openspec/changes/realtime-booking-chat → archive/2026-05-29-realtime-booking-chat
- 新增 openspec/specs/booking-chat/spec.md（即時訊息完整規格）
- 新增 openspec/specs/user-presence/spec.md（Presence Channel 規格）
- 更新 openspec/specs/booking-lifecycle/spec.md（補訊息頻道語義與封存 Scenario）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/archive/2026-05-29-realtime-booking-chat/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-05-26