Add external tunnel provider support (localtunnel.me)

[kernelshard]

Summary

Implement external tunnel provider to expose local server to the internet using localtunnel.me

Motivation

Currently expose only creates tunnels on localhost. Users need to expose their local development server to the internet for:

• Testing webhooks (Stripe, GitHub, etc.)
• Sharing work with clients/team
• Mobile device testing
• Remote debugging

Proposed Solution

Add localtunnel.me provider that:

• Requests public URL from localtunnel.me (no signup required)
• Establishes TCP connection pool (10 connections)
• Proxies external traffic to local server
• Handles reconnection on failure

Implementation Plan

• Create Provider interface in internal/provider/
• Implement LocalTunnel provider
• HTTP request to get tunnel info
• TCP connection pool management
• Traffic proxying logic
• Graceful shutdown
• Unit tests
• Integration with CLI

Acceptance Criteria

• expose tunnel gets public URL from localtunnel.me
• External requests reach local server
• Handles connection drops gracefully
• 80%+ test coverage
• Documentation updated