Add `specscore consilium verdict` subcommand

[trakhimenok]

Context

The sidekick-consilium Feature in synchestra-io/specstudio-skills ships the contract for a new deterministic CLI arbiter that turns 9 typed votes (from a 9-role panel) into a verdict. REQs specscore-consilium-verdict-subcommand, arbiter-gate-rules, arbiter-reproducibility, and roster-validation define the contract.

Tracking stub in the producing repo: spec/plans/sidekick-consilium-arbiter-companion.md.

What needs to ship

A new subcommand specscore consilium verdict accepting --votes, --roster, --gate, --seed. Applies the 13-step gate algorithm from REQ arbiter-gate-rules and emits a structured YAML verdict + rule_trace to stdout. Snapshot-testable: same inputs always produce identical output.

Acceptance

A 13-step gate algorithm implementation with snapshot tests over fixtures covering each rule path: adversary-veto, low-confidence-abstain veto, strict-gate pass, supermajority pass (when require_all_builders=false), median-confidence-fails, cost-ceiling-fails, complexity-ceiling-fails, builders-majority-reject, and the fall-through to needs-human-review.

Cross-reference

• Feature spec: spec/features/sidekick-consilium/README.md REQs 23–25
• Companion plan: spec/plans/sidekick-consilium-arbiter-companion.md