feat: add output file management for per-node command results

[inureyes]

Summary

• Implement output file management feature to save command results to separate files per node
• Add --output-dir option to specify where to save outputs
• Refactor execute_command to use parameter struct to fix clippy warnings

Changes

• Save stdout and stderr to separate files per node with timestamped filenames
• Create error files for failed connections or command executions
• Generate empty marker files when commands produce no output
• Create execution summary file with overall success/failure counts
• Add metadata headers to all output files (command, host, user, exit status, timestamp)
• Update documentation in README.md and manpage
• Add chrono dependency for timestamp generation
• Refactor execute_command function to use ExecuteCommandParams struct

File Structure

When using --output-dir, the following files are created:

output-dir/
├── hostname1_YYYYMMDD_HHMMSS.stdout   # Standard output
├── hostname1_YYYYMMDD_HHMMSS.stderr   # Standard error (if any)
├── hostname2_YYYYMMDD_HHMMSS.error    # Connection/execution errors
├── hostname3_YYYYMMDD_HHMMSS.empty    # Marker for no output
└── summary_YYYYMMDD_HHMMSS.txt        # Overall execution summary

Usage Example

bssh -c production --output-dir ./results "ps aux"

Test Plan

• Tested output file creation with successful commands
• Tested stderr capture for commands with error output
• Tested empty file markers for commands with no output
• Tested error file creation for failed connections
• Verified summary file generation with correct counts