refactor: Extract CLI and commands into separate modules

Create cli.rs and commands.rs:
- cli.rs: parse_arg_matches() and all clap configuration (446 lines)
- commands.rs: run() and parse_sort_option() command handling (727 lines)

Reduced main.rs from 1,222 lines to 68 lines (94% reduction).
Main.rs now contains only:
- Module declarations
- Helper functions (executable_name, check_gh_cli_installed)
- run_app and main

All 52 tests pass.

Author: dashed