shipping: define local review fallback for stuck artifact-free webhook reviews

[teslamint]

Problem

Cogvault PR #34 had an external webhook review stuck in progress for over seven hours without review artifacts. A local CodeRabbit CLI review produced findings in under two minutes.

The existing skipped-review guidance does not define a standing fallback for this state.

Requested change

Define a bounded fallback in the shipping workflow for a required external review that remains in progress without artifacts.

Acceptance criteria

• Distinguish stuck, skipped, failed, and completed reviews using artifacts and execution state.
• Define a bounded wait and an explicit transition to a supported local reviewer.
• Retain the reviewed revision, command, completion result, findings, and their dispositions.
• Invalidate stale review evidence when the reviewed change changes.
• Never treat a timeout or fallback failure as a waiver or completed review.
• Add workflow regression coverage for successful fallback, unavailable CLI, incomplete review, and changed revision.

Evidence and scope

• Cogvault retrospective
• Existing guidance

Compound-loop owns the reusable workflow. Cogvault retains the originating evidence. This issue does not request changes to Cogvault runtime code.