feat: add SSH-compatible command-line interface and fix localhost connectivity

[inureyes]

Summary

This PR adds SSH-compatible command-line interface to bssh, enabling it to work as a drop-in replacement for the standard SSH client while maintaining its powerful multi-server capabilities.

Key Changes

SSH Compatibility Mode

• Added support for SSH-style command syntax: bssh user@host [command]
• Implemented SSH-compatible options (-l, -p, -o, -F, -Q, etc.)
• Auto-detection of single-host vs multi-server mode
• Parse SSH URIs: ssh://user@host:port

Command-Line Options

• Changed cluster option from -c to -C to avoid conflicts with SSH
• Added -l for login username (SSH-compatible)
• Added -p for port specification (moved parallel to --parallel only)
• Added -o for SSH options (e.g., -o StrictHostKeyChecking=no)
• Added -F for SSH config file specification
• Added -Q for querying SSH capabilities (cipher, kex, mac, etc.)
• Added -t/-T for TTY control
• Added -4/-6 for IP version forcing

Authentication & Connection Fixes

• Fixed localhost connectivity issue: bssh localhost now works like ssh localhost
• Improved authentication fallback to try default SSH keys (~/.ssh/id_rsa, id_ed25519, etc.)
• Skip SSH agent auto-detection to avoid failures with empty agents
• Only use SSH agent when explicitly requested with -A flag

Interactive Mode

• Auto-switch to interactive mode when no command is provided (like SSH)
• Single-host SSH connections now properly start interactive sessions
• Fixed issue where bssh tried to execute "bash || sh" as a command

Usage Examples

# SSH-compatible single host mode
bssh user@hostname                    # Interactive shell
bssh user@hostname "uptime"           # Execute command  
bssh -p 2222 -i ~/.ssh/key user@host  # Custom port and key
bssh -o StrictHostKeyChecking=no host # SSH options

# Multi-server mode (existing functionality)
bssh -C production "uptime"           # Cluster execution
bssh -H "host1,host2,host3" "df -h"   # Multiple hosts

Testing

All the following now work correctly:

• ✅ bssh localhost - Interactive session without specifying key
• ✅ bssh localhost whoami - Command execution
• ✅ bssh user@host - SSH-style connection
• ✅ bssh -i ~/.ssh/key host - Explicit key specification
• ✅ bssh -Q cipher - Query SSH capabilities

Related Issues

Created follow-up issues for remaining SSH features:

• feat: Add SSH port forwarding support (-L, -R, -D options) #21: Port forwarding support (-L, -R, -D)
• feat: Add SSH jump host support (-J option/ProxyJump) #22: Jump host support (-J option/ProxyJump)
• feat: Add SSH configuration file parsing (-F option) #23: SSH configuration file parsing
• feat: Implement true PTY allocation for interactive mode #24: True PTY allocation for interactive mode

Breaking Changes

• Cluster option changed from -c to -C (uppercase)
• Parallel option -p is now --parallel only (to support SSH port option)

Migration Guide

For existing users:

• Replace -c cluster with -C cluster in scripts
• Replace -p N with --parallel N for parallelism control