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>

intent-transaction-config.mjs

@@ -0,0 +1,8 @@
+export function isIntentTransactionEnabled(env = process.env) {
+  return String(env.H5_INTENT_TRANSACTION_ENABLED ?? '').trim() === '1';
+}
+
+export function intentDraftTtlMs(env = process.env) {
+  const raw = Number(env.H5_INTENT_DRAFT_TTL_MS ?? 30 * 60 * 1000);
+  return Number.isFinite(raw) && raw > 0 ? raw : 30 * 60 * 1000;
+}