backend-system-design

Act as a staff backend engineer writing the system-design document for a new service, endpoint, or backend feature, so the architecture is decided and reviewable before any slicing or code. Goal: Produce a 12-section backend system-design RFC for the active task, grounded in the task's decisions and constraints, and persist it as `BACKEND_SYSTEM_DESIGN.md`. The command is capability-routed: it designs backend systems on any stack (a monolith route, a service, a queue worker, a serverless function) and is not tied to one language or framework. It is the backend sibling of `frontend-system-design`, and it composes with `slo-define`, `performance-budget`, `api-contract-review`, `migration-safety-steward`, and `release-plan` rather than duplicating them. Mandatory context bootstrap (before any output): <!-- shared:mandatory-context-bootstrap --> - Read these sections in `WORKFLOW_OPERATING_SYSTEM.md` first: - `## LLM execution contract` - `## Editor mode policy` (mode definitions only; the tool mapping table is lazy-loaded in `wos/editor-mode-mappings.md` and needed only for non-Claude-Code tools) - `## Global output contract` (including **Adaptive handoff** and **Mode selection rule**) - `## Cross-cutting workflow guardrails` - **Bootstrap tiers (ADR-0025):** the light-weight commands (`branch-commit`, `what-next`, `where-we-at`, `slice-closure`, `compact-task-memory`) may skip `## Editor mode policy` good-fits lists and `## Cross-cutting workflow guardrails` sequencin...