feat: fully interactive copilot-style installer

Complete rewrite of install.ps1 as an interactive setup assistant:
- Scans system and shows what's installed vs missing
- Asks Y/n for each dependency (Node.js, Git, gh CLI)
- Asks Y/n for each AI provider CLI (Codex, Claude, Gemini)
- Asks before downloading/installing T3 Code MSI
- Shows live winget progress during installs
- Friendly "T3" bot personality with colored output
- Same command works for install and update

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hlsitechio