Feat v0.9.2 Starlark converter consolidation + snapshot cleanup (#75)

Consolidate duplicate Go↔Starlark converters into sandbox/convert.go
and snapshot parsers into models/snapshot.go. Standardize snapshot
creation on wrapped format. Net -393 lines across 21 files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/models/snapshot.go

@@ -0,0 +1,25 @@
+package models
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// ParseSnapshotStages parses a version snapshot into a slice of
+// WorkflowStage.  It handles both the wrapped format
+// {"stages": [...], "workflow": {...}} and the legacy flat array [...].
+func ParseSnapshotStages(raw json.RawMessage) ([]WorkflowStage, error) {
+	// Try wrapped format first
+	var wrapped struct {
+		Stages []WorkflowStage `json:"stages"`
+	}
+	if err := json.Unmarshal(raw, &wrapped); err == nil && len(wrapped.Stages) > 0 {
+		return wrapped.Stages, nil
+	}
+	// Fallback to flat array
+	var stages []WorkflowStage
+	if err := json.Unmarshal(raw, &stages); err != nil {
+		return nil, fmt.Errorf("corrupt version snapshot: %w", err)
+	}
+	return stages, nil
+}