feat(cli): add interactive multi-select for init command

[dacsang97]

Summary

Replace single-select prompt with multi-select UI using @clack/prompts, allowing users to install for multiple AI assistants in one command.

Changes

• Add @clack/prompts dependency for modern CLI UI
• Replace prompts single-select with p.multiselect()
• Show (detected) hints next to auto-detected AI assistants
• Pre-select detected agents by default
• Install for multiple selected agents in one run

Before

? Select AI assistant to install for: (Use arrow keys)
❯ Claude Code
Cursor
Windsurf

After

◆ Select AI assistants to install for:
│ ◼ Claude Code      (detected)
│ ◼ Cursor           (detected)
│ ◻ Windsurf
│ ◻ GitHub Copilot

Testing

cd cli && bun install && bun run build
./dist/index.js init