[Workflows] Enforce CodeRabbit before Copilot on PRs

[ashleyshaw]

Title: [Workflows] Enforce CodeRabbit before Copilot on PRs
Labels: area:workflows, v0.2.0, reviewer
Summary: Guarantee CodeRabbit completes before Copilot runs; fail if the order inverts.
Acceptance Criteria:

reviewer.yml defines job dependency: coderabbit → copilot.

Required checks configured so copilot cannot succeed before coderabbit.

Annotations emitted when order violated.
Impact/Risk: Keeps reviews consistent and useful.
Dependencies/Links: Part-1 audit.
Owner: Workflows
Telemetry: % PRs with correct reviewer order.