feat: add comprehensive configuration management for interactive mode

[inureyes]

Summary

This PR adds comprehensive configuration management for interactive mode, allowing users to configure interactive settings both globally and per-cluster through the configuration file.

Changes

Configuration Features

• InteractiveConfig struct with configurable options:
  • default_mode: Choose between single_node or multiplex
  • prompt_format: Customizable prompt with variable substitution
  • history_file: Command history persistence location
  • show_timestamps: Optional timestamps in output
  • work_dir: Initial working directory
  • broadcast_prefix and node_switch_prefix: Customizable command prefixes
  • colors: Node-specific output coloring
  • keybindings: Future-ready keybinding configuration

Configuration Management

• Global interactive settings in config.yaml
• Per-cluster configuration overrides
• CLI arguments take precedence over configuration file settings
• Configuration save/update functionality with preferences persistence
• Priority chain: CLI args → Cluster config → Global config → Defaults

Example Configuration

# Global settings
interactive:
  default_mode: multiplex
  prompt_format: "[{node}] $ "
  show_timestamps: true
  broadcast_prefix: "!all "
  node_switch_prefix: "!"
  colors:
    node1: red
    node2: blue
  keybindings:
    switch_node: "Ctrl+N"
    broadcast_toggle: "Ctrl+B"
    quit: "Ctrl+Q"

# Cluster-specific overrides
clusters:
  production:
    interactive:
      default_mode: single_node
      prompt_format: "prod> "
      work_dir: /var/www

Documentation Updates

• Updated CLI --help to document configuration override behavior
• Enhanced README.md with detailed configuration examples
• Updated manpage with complete interactive mode documentation
• Added notes about customizable command prefixes

Usage Examples

1. Use configuration file settings:

   bssh -c production interactive
   # Uses production cluster's interactive settings

2. Override with CLI arguments:

   bssh -c production interactive --single-node --prompt-format "custom> "
   # CLI args override config file settings

3. Customized command prefixes:

   interactive:
     node_switch_prefix: "@"
     broadcast_prefix: "@all "

   Now use @Node1, @ALL commands instead of !node1, !broadcast

Testing

• Added unit tests for configuration parsing and merging
• Added tests for configuration save/load functionality
• Verified configuration priority chain works correctly
• Tested per-cluster configuration overrides

Breaking Changes

None - all changes are backward compatible. Default behavior remains unchanged if no configuration is provided.