docs: add DAG and worker system specifications
DAG.md describes the two-tier dependency graph (BOM DAG + feature DAG),
node/edge data model, validation states, dirty propagation, forward/backward
cone queries, DAG sync payload format, and REST API.
WORKERS.md describes the general-purpose async compute job system: YAML job
definitions, job lifecycle (pending→claimed→running→completed/failed),
runner registration and authentication, claim semantics (SELECT FOR UPDATE
SKIP LOCKED), timeout enforcement, SSE events, and REST API.
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