feat: add tunnel support (ngrok, Cloudflare Tunnel)

[sergical]

Summary

Adds support for external tunnel providers (ngrok, Cloudflare Tunnel) to expose local dev servers to the internet. Closes #43.

Session trace: https://www.traces.com/s/jn7c9rx9wqywxp9a77b265qtg582p96p

What changed

Three levels of integration, each independently useful:

1. Single-app passthrough (zero config)

When one app is running and a request arrives with a non-.localhost Host header (e.g. from ngrok forwarding), portless auto-routes it to the single registered app. This means the simplest tunnel workflow just works:

portless myapp next dev       # Terminal 1
ngrok http 1355               # Terminal 2 -- auto-routed

2. Hostname aliasing (portless tunnel map/unmap/list)

For multi-app setups, explicitly map an external tunnel hostname to a portless route:

portless tunnel map myapp abc123.ngrok-free.app
portless tunnel unmap abc123.ngrok-free.app
portless tunnel list

Aliases are stored in aliases.json in the state directory and watched for changes by the proxy.

3. Managed tunnels (--tunnel flag)

Portless can start the tunnel for you, auto-register the alias, and pass the tunnel URL to the child process:

portless myapp --tunnel ngrok next dev
portless run --tunnel cloudflare next dev

The child process receives PORTLESS_TUNNEL_URL alongside the existing PORTLESS_URL. Tunnel cleanup (process kill + alias removal) happens automatically on exit.

Files changed

File	Changes
proxy.ts	findRoute() extended with alias lookup + tunnel passthrough fallback
types.ts	Added getAliases to ProxyServerOptions
routes.ts	Alias storage: loadAliases, addAlias, removeAlias, removeAliasesForRoute
tunnel.ts	New: TunnelProvider/TunnelInstance interfaces, ngrok + cloudflare providers
cli.ts	--tunnel flag, PORTLESS_TUNNEL env var, handleTunnel command, alias watching in proxy startup
index.ts	Re-exports tunnel.ts
README.md	New "Tunnels" section, updated commands/options/env vars
SKILL.md	Tunnel docs, updated CLI reference and env vars tables
Tests	36 new tests across proxy.test.ts, routes.test.ts, tunnel.test.ts, cli.test.ts

Testing

• 323 tests passing (36 new)
• Typecheck clean
• Lint clean
• Zero new runtime dependencies

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