feat: add user preferences system

[Freewheelin]

Add comprehensive user preferences system to streamline CLI workflows and customise output formatting.

Features

Server Creation Defaults

• Set default values for --region, --size, --image, and other server creation parameters
• Create servers with zero arguments once preferences are configured
• Example: bl server create uses configured defaults while allowing command-line overrides

Output Formatting Preferences

• Customise default columns for list commands (servers, images, domains, etc.)
• Set once with format-servers, format-images, etc.
• Applies automatically to all list commands while allowing per-command --format overrides

Terminal Settings

• Configure terminal width for help text formatting
• Set with terminal-width preference (numeric value or null for auto-detection)

Management Commands

• bl preferences set <key> <value> - set or update a preference
• bl preferences get [key] - view current preferences
• bl preferences show [group] - display preferences with contextual help and available options
• bl preferences set <key> null - reset to built-in defaults

Enhanced User Experience

• preferences show server create displays all server creation defaults with command preview
• Contextual help shows valid values and examples
• Integration with existing configuration system (respects command-line > env > preferences > defaults priority)

Commands Added

• preferences get
• preferences set
• preferences show
• server create (wrapper that applies preferences to generated command)

Documentation

• Comprehensive preferences guide in PREFERENCES.md
• Updated README.md with preferences section and examples
• Inline help for all preference keys

Use Case

Before:

bl server create --name web-01 --size std-min --image ubuntu-24.04 --region syd
bl server create --name web-02 --size std-min --image ubuntu-24.04 --region syd

After:

# Set once
bl preferences set default-region syd
bl preferences set default-size std-min
bl preferences set default-image ubuntu-24.04

# Use everywhere
bl server create
bl server create --name web-01

This feature reduces repetitive typing for users who frequently create servers with similar configurations or prefer customised list output formats.