Package metric regression gates in CI

[jflowers]

Summary

Add CI gates that detect regression in package-level coupling metrics between the base branch and the PR, failing the build when structural quality degrades beyond tolerance.

RFC: https://github.com/orgs/unbound-force/discussions/483 — Group 2d
Phase: P2 (Accessibility)

Context

Once vibe-check analyze produces package-level metrics, we need CI enforcement that prevents PRs from degrading those metrics. This adds regression detection as a first-class vibe-check capability.

Implementation

Regression Detection

• Run vibe-check analyze on both the base branch and the PR branch
• Compare per-package metrics: Instability (I), Distance (D), LCOM, Circular Deps
• Fail if any metric regresses beyond a configurable tolerance

CLI Flags

vibe-check analyze ./... \
  --regression-base=origin/main \
  --max-instability-delta=0.05 \
  --max-distance-delta=0.05 \
  --no-new-circular-deps

Output

• Table of packages with metric deltas
• Clear indication of which packages regressed and by how much
• JSON output with regression section for programmatic consumption

Acceptance Criteria

• --regression-base flag compares metrics between branches
• Configurable delta thresholds for instability, distance, cohesion
• --no-new-circular-deps flag detects newly introduced cycles
• Clear CI output showing regression details
• JSON output includes regression section conforming to metrics.ModuleGraph schema
• Non-zero exit code on regression beyond tolerance
• Integrated into CI workflow

Dependencies

• Depends on: vibe-check analyze — Go-native package-level coupling metrics #2 — vibe-check analyze command
• Related: divisor-entropy agent — structural delta enforcement #3 (divisor-entropy agent)