feat: Add Cloudflare Tunnel provider support

[kernelshard]

Summary

Add Cloudflare Tunnel (cloudflared) as second provider alongside LocalTunnel.

Motivation

• More reliable than LocalTunnel for some use cases
• Extensible provider interface

Implementation

• Provider interface + Cloudflare impl
• CLI --provider/-P flag
• Tests (90%+ coverage)
• Docs + examples

Acceptance Criteria

• expose tunnel -P cloudflare -p 3000 works
• Graceful Ctrl+C shutdown
• Both providers switchable via flag/config