feat: git snapshot + revert before each EXECUTE retry

[dougss]

Summary

Before each EXECUTE attempt in retry loop, snapshot worktree state. On validation failure, revert to clean state before next attempt — prevents executor from inheriting broken code.

Current State

• runPipeline() loops EXECUTE↔VALIDATE with previousError text only
• No state cleanup between retries — executor starts from dirty state of failed attempt

Plan

1. Create src/core/snapshot.ts:
   • createSnapshot(repoPath): string — lightweight git tag
   • restoreSnapshot(repoPath, ref): void — git checkout + clean
   • cleanupSnapshot(repoPath, ref): void — remove tag
2. In runPipeline(), snapshot before retry loop, restore on failure
3. Cleanup on success or final failure

Files

• src/core/snapshot.ts (NEW)
• src/core/pipeline.ts