Why: Q2 (topology library) landed in Agent-Squad main across WP1-4
(12 builtin topologies with scripted-backend conformance); backend consumes
squad via squadadapter and must track the completed surface.
Evidence: GOWORK=off go build ./... and go test ./... -count=1 green against
squad bcc8d08 (SELF); backend CI on the PR head is authoritative (GATE).
Impact: nested deps/agent-squad c462bc4 -> bcc8d08 (additive squad API:
topology registry, RunOptions knobs, mailbox/handoff/board state); no
backend source changes required.
Refs: Q2 plan (2026-10-07-agent-stack-q2q3-plans.md); Dev task card
.task/feature-agent-stack-q2.md