Improve rgit install guidance mode picker

[sunjiajunsunjiajun]

Summary

• Add an interactive guidance mode picker for rgit install <platform>.
• Keep the existing numbered prompt as a silent fallback.
• Preserve script-friendly behavior for --guidance and non-TTY installs.

Details

• Supports up/down navigation, Enter selection, and 1/2/3 shortcuts.
• Keeps prompt UI on stderr so install JSON remains on stdout.
• Handles Ctrl+C as a clean cancellation with exit code 130.
• Gates the selector on Windows ANSI support and falls back when unsupported.

Validation

• python -m pytest -q
• Result: 310 passed, 2 skipped

Closes #16

[sunjiajunsunjiajun]

Mac:

Windows:

[sunjiajunsunjiajun]

@Lum1104 FYI: this PR is mainly an install UX improvement for the rgit install guidance mode picker. We may want to include it in a future package release, but there is no urgency to cut a new package immediately just for this.