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>

task-unified-config.mjs

@@ -0,0 +1,3 @@
+export function isUnifiedTasksEnabled(env = process.env) {
+  return String(env.H5_UNIFIED_TASKS_ENABLED ?? '').trim() === '1';
+}