Agent Skills: dual-review

Run the ca dual-model review (Codex second opinion + blind Claude review + Claude synthesis) standalone on any PR — no implement loop required. Use to get a Claude-and-Codex joint review of a pull request, to re-review a PR after the ca loop finished, or when the user says "dual review this PR", "review with both Claude and Codex", or invokes /ca:dual-review. Emits the same ca_claude_review.v1 verdict as the loop's final review.