docs: record the deferred promotion-window invariant and test-isolation rules

Add the shared promotion-window EXCLUDE-constraint idea to the deferred designs
so the flash/group overlap ordering problem has a recorded structural fix.

Note in AGENTS.md that suite assertions must be scoped to fixtures the test
created, that a list endpoint warrants two consecutive green runs, and that a
piped test command hides the real exit code.

docs/TBD-marketing.md

@@ -116,6 +116,13 @@ transaction rules before implementation.
 - [ ] **Withdrawals / 提现** — model a payout request with account, amount minor, fee minor,
   destination snapshot, and `requested → approved | rejected | paid` transitions. Funds must
   be frozen and released or debited atomically with the transition.
+- [ ] **Promotion window exclusivity as a database invariant** — flash sales and group buying
+  currently keep one SKU out of overlapping activity windows with cross-table queries, and the
+  flash-sales guard stays inert until the group-buying table exists. A shared
+  `promotion_windows(sku_id, kind, starts_at, ends_at)` table with
+  `EXCLUDE USING gist (sku_id WITH =, tstzrange(starts_at, ends_at) WITH &&)` would make an
+  overlap impossible to insert and remove the creation-order problem. Deferred because it
+  retrofits the archived flash-sales schema; revisit if a third timed activity type appears.
 
 ## Deliberately out of scope — does not block deleting this file