feat: add export/import functionality for backup and migration

Implemented roadmap #4: Export/import functionality.

Features:
- export <file> <key1> [key2...] - Export specified keys to JSON
- export-list <keyfile> <output> - Bulk export from key list file
- import <file> - Import keys from JSON file
  --skip-existing (default) - Skip keys that already exist
  --overwrite - Overwrite existing keys

Export file format (v1.0):
{
  "version": "1.0",
  "entries": [
    {
      "key": "users/alice",
      "uuid": "...",
      "timestamp": 1234567890,
      "data": {...}
    }
  ]
}

The format preserves UUIDs and timestamps for audit trails.
Export shows progress with ✓/✗/⊘ symbols for success/fail/not-found.
Import checks for existing keys and respects skip/overwrite flags.

Use cases:
- Backup: export-list keys.txt backup.json
- Migration: import backup.json --overwrite
- Selective sync: export dest.json key1 key2

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd_system.go

@@ -60,6 +60,13 @@ Group Management:
   group add-member <uuid> <user-uuid> - Add member to group
   group remove-member <uuid> <user-uuid> - Remove member from group
 
+Export/Import:
+  export <file> <key1> ...   - Export keys to JSON file
+  export-list <keyfile> <output> - Export keys listed in file
+  import <file>              - Import keys from JSON file
+    --skip-existing          - Skip existing keys (default)
+    --overwrite              - Overwrite existing keys
+
 Batch Operations:
   batch <file>               - Execute commands from file
   source <file>              - Alias for batch