Why: N1 WP5/WP6 library side - the gate decision logic, artifact
checks, verdict parsing and progress-reminder throttling move from
the product into a gates package behind ports, and the native backend
gains a contract form so conformance has a real implementation to
drive.
Evidence: go vet clean; go test ./... 27 packages ok, including the
conformance suite (succeeded/descriptor/error-sideeffects) against
the real backend.
Impact: new runtime/gates package (ClaimGate, CheckArtifacts,
ParseVerdict, ProgressReminder, shared caps); adapters/native.Backend
gains Contract() returning an exec.ContractBackend wrapper - the
legacy facade keeps its signature so squad consumption is untouched.
Product texts (fix prompts, verifier prompts) stay injected by hosts.
Refs: Documents proposal 2026-10-07-agent-stack-n1q1i1-plans.md WP5/WP6
