feat: Goal-Backward Verification for Shepherd

## Summary

Strengthen Shepherd to verify observable truths from the original request, check artifact depth (exists vs substantive vs wired), detect stubs, and support re-verification of previously failed items.

## Context

Identified in Anvil competitive analysis (#114). Enhances #27 (Shepherd hooks).

## Dependencies

- #27 (Shepherd hooks for subagent orchestration)

## Effort

Medium