docs: API architecture ADRs, tech spec, and marketing capability tracker

Persist the modular-monolith decision (ADR 0001 handler/service/repo,
ADR 0002 keep REST) with the companion tech spec and the api-architecture
OpenSpec capability. Replace the finished mock-migration tracker with
docs/TBD-marketing.md listing the backend-less marketing domains still on
fixtures (coupons, favorites, account stats, seckill, collective, integral,
reviews). README and AGENTS.md point at the new docs.

docs/adr/README.md

@@ -0,0 +1,14 @@
+# Architecture Decision Records
+
+ADRs in this directory record **why** the Rust API (`apps/api`, crate `vmall-api`) is structured the way it is. They are written in English and do not replace OpenSpec capability specs (`openspec/specs/`), which describe **what** HTTP behavior clients may rely on.
+
+| ID | Title | Status |
+|----|--------|--------|
+| [0001](0001-rust-api-modular-monolith.md) | Modular monolith with handler / service / repository | Accepted |
+| [0002](0002-keep-http-rest-not-graphql.md) | Keep HTTP REST; do not replace the API with GraphQL | Accepted |
+
+Template (MADR-inspired): Context → Decision → Consequences → Alternatives.
+
+New ADRs: next unused number, `NNNN-kebab-title.md`, Status `Proposed` until accepted.
+
+Companion: [tech spec — Rust API](../tech-specs/rust-api.md), [OpenSpec — api-architecture](../../openspec/specs/api-architecture/spec.md).