Feature: add `h8 set port` command to add/remove ports on existing apps

## Description

Currently there is no way to modify the service ports of an existing app. To add or remove a port, you must delete the entire app and recreate it — which loses disk attachments, env vars, and other configuration.

## Proposed Command

```bash
h8 set port APP PORTS...
```

Where `PORTS` is in the same `svcPort:containerPort` format as `app create --port`.

Examples:
```bash
h8 set port myapp 8080:8080 # set/replace all ports
h8 set port myapp 3000:3000 # remove SSH, keep only HTTP
h8 set port myapp 3000:3000 2222:22 # add SSH back
```

## Workaround

Currently requires delete + recreate (destructive): `h8 app delete` then `h8 app create` with the desired ports, re-attach disk, re-set all env vars.

Start with the existing `app create --port` command and its `svcPort:containerPort` parsing, then trace how `h8 app delete` and `h8 app create` update an app. Add the `h8 set port APP PORTS...` entry point so ports on an existing app can be replaced or updated without losing other configuration; verify the documented examples and removal behavior.

Written by the indexing model from the issue text.