feat(wechat): add Intent Transaction Layer with unified task schema

Introduce Draft → Confirm → Commit flow for WeChat schedule intents behind
feature flags, plus h5_tasks dual-write/read aggregation and rollout scripts
so reminders and automations get explicit user confirmation before persisting.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -185,6 +185,13 @@ H5_PUBLIC_BASE_URL=http://127.0.0.1:5173
 # H5_SCHEDULED_TASK_MAX_ATTEMPTS=3
 # H5_SCHEDULED_TASK_EXECUTION_TIMEOUT_MS=900000
 
+# Intent Transaction Layer（Phase A：Draft → Confirm → Commit；默认关闭）
+# H5_INTENT_TRANSACTION_ENABLED=1
+# H5_INTENT_DRAFT_TTL_MS=1800000
+
+# Unified Task Schema（Phase B：读聚合 + Commit 双写；默认关闭）
+# H5_UNIFIED_TASKS_ENABLED=1
+
 # H5 Session stream replay
 # 默认 0：session SSE 仍纯透传 goosed；设 1 时 Portal 持久化 session 事件并支持 Last-Event-ID 重连补发。
 # MEMIND_SESSION_STREAM_REPLAY=0