feat: add batch command execution from files

Implemented roadmap #3: Batch operations from file.

Features:
- batch <file> or source <file> - Execute commands from script file
- Skips empty lines and comments (lines starting with #)
- Shows line numbers during execution for easy debugging
- Blocks exit/quit commands in batch mode (won't exit shell)
- Displays execution summary with counts

Implementation:
- Refactored main loop to extract executeCommand() method
- Created cmd_batch.go with batch file processor
- Reads file line by line, processes each command
- Summary shows total lines, executed count, and skipped count

Example batch file:
  # Connect and setup
  connect http://localhost:8080
  auth <token>

  # Batch insert
  put key1 '{"data":"value1"}'
  put key2 '{"data":"value2"}'

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

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

cmd_system.go

@@ -60,6 +60,11 @@ Group Management:
   group add-member <uuid> <user-uuid> - Add member to group
   group remove-member <uuid> <user-uuid> - Remove member from group
 
+Batch Operations:
+  batch <file>               - Execute commands from file
+  source <file>              - Alias for batch
+    --continue-on-error      - Continue execution even if commands fail
+
 System:
   help                       - Show this help
   exit, quit                 - Exit shell