refactor(sandbox): extract formatters into separate module

Move formatting utilities to sandbox-formatters.ts:
- formatStatus, formatSimpleStatus, formatImageMatch, formatAge
- countRunning, countMismatches helper functions
- ContainerItem type definition

Improves modularity and reusability. 49 LOC.

src/commands/sandbox-formatters.ts

@@ -0,0 +1,49 @@
+/**
+ * Formatting utilities for sandbox CLI output
+ */
+
+export function formatStatus(running: boolean): string {
+  return running ? "🟢 running" : "⚫ stopped";
+}
+
+export function formatSimpleStatus(running: boolean): string {
+  return running ? "running" : "stopped";
+}
+
+export function formatImageMatch(matches: boolean): string {
+  return matches ? "✓" : "⚠️  mismatch";
+}
+
+export function formatAge(ms: number): string {
+  const seconds = Math.floor(ms / 1000);
+  const minutes = Math.floor(seconds / 60);
+  const hours = Math.floor(minutes / 60);
+  const days = Math.floor(hours / 24);
+
+  if (days > 0) return `${days}d ${hours % 24}h`;
+  if (hours > 0) return `${hours}h ${minutes % 60}m`;
+  if (minutes > 0) return `${minutes}m`;
+  return `${seconds}s`;
+}
+
+/**
+ * Type guard and counter utilities
+ */
+
+export type ContainerItem = {
+  running: boolean;
+  imageMatch: boolean;
+  containerName: string;
+  sessionKey: string;
+  image: string;
+  createdAtMs: number;
+  lastUsedAtMs: number;
+};
+
+export function countRunning<T extends { running: boolean }>(items: T[]): number {
+  return items.filter((item) => item.running).length;
+}
+
+export function countMismatches<T extends { imageMatch: boolean }>(items: T[]): number {
+  return items.filter((item) => !item.imageMatch).length;
+}