feat: interactive TUI mode

[nmhjklnm]

Summary

When running cac with no arguments, enter an interactive TUI mode instead of showing help.

Features

• Environment list with arrow key navigation, Enter to activate
• Creation wizard: guided input for name, proxy, version
• Status panel: current env + proxy connectivity + version info

Technical options

• Simple: bash select/read — minimal dependency
• Rich: Node.js with inquirer or ink — better UX but adds dependency

Current behavior

cac with no args shows cmd_help output.

Proposed behavior

cac              # → interactive TUI
cac help         # → help text (unchanged)