Document current architecture (web + api + Stargate runtime + artifact platform)

## Why
No up-to-date architecture reference exists for how the pieces fit together: `apps/web` (Next.js), `apps/api` (NestJS + Rockets SDK), the Stargate runtime, and the generic artifact/micro-app platform (`modules/workflows`).

## Scope
- Diagram (mermaid or excalidraw, committed to the repo) covering:
- `apps/web` routes (`/login`, `/artifacts`, `/report/[name]`) and how they call the API.
- `apps/api`: `AppModule` wiring (`RocketsModule.forRoot`, `AccessControlModule`), `modules/workflows` (Flows/Records/MCP controllers, `ArtifactWorkspaceService`, `ArtifactRecordsService`, `ArtifactSyncService`), `modules/stargate` (catalog/execution services, `STARGATE_RUNTIME` port), `StargateRuntimeModule` wrapping `@stargate/server/nest`.
- The `.stargate/` filesystem workspace (`flows/`, `ui/`, `schema/`, `mcp.json`) as the artifact source of truth.
- External deps: Microsoft Entra ID (auth), OpenAI (flow steps), the `@stargate/*` sibling packages.
- Short doc (README or `docs/architecture.md`) explaining the artifact lifecycle: install (HTTP `POST /flows` or MCP `install_artifact`) → run (`/flows/:name/run`) → optional schema-driven persistence (`/apps/:app/records`).

## Out of scope
Redesign of any of the above — this is documentation of what exists today.