Add interactive config generator (updo init)

[pgagnidze]

Most modern CLI tools provide an interactive way to generate initial configuration files. Currently, users must manually create TOML files or copy examples, which creates friction during initial setup.

Current behavior

Users need to:

1. Find example configuration files
2. Manually edit TOML syntax
3. Reference documentation for available options
4. Trial and error to get settings right

Proposed feature

Add updo init command that guides users through configuration creation with:

• Interactive prompts for common settings (targets, refresh interval, timeout)
• Option selection for HTTP methods, regions, notification settings
• Validation during input to prevent invalid configurations
• Generated TOML file ready to use

Benefits

• Reduces time to first successful monitoring from minutes to seconds
• Eliminates common configuration mistakes
• Follows CLI tool conventions users expect
• Improves adoption for new users

Implementation notes

Could use a library like survey/promptui for interactive prompts and generate valid TOML output.