Restore !joinas functionality for friendly private games

### Description

BAR used to have the capability to use the !joinas command from a lobby to add another person to the game after it's running.

At some point this stopped working: now it automatically forces spec instead. As I understand it this was intentional to prevent people abusing it in public lobbies, which is why this is a feature request instead of a bug report.

This was a really impressive and useful feature to join my friends when they're 10-20 minutes into a barbarian or raptor game, and I don't see any harm in allowing it for private games.

### Why?

Allows people to join their friends when they're mid-game in a non-competitive mode, rather than waiting 30+ minutes for their game to end or forcing them to restart.

### What area of the game does this feature request pertain?

Other

### Other (optional)

I understand the feature isn't going to restored as it was, but restricting it to games with friends or private matches in general seems like it would curb any ability to abuse it.

Start by locating the existing !joinas command handling and the logic that currently forces spectators. Trace how the game distinguishes private or friendly matches from public lobbies, then clarify the intended safety rules and acceptance criteria with maintainers; done means the permitted non-competitive cases allow the player to join an ongoing game without restoring public-lobby abuse.

Written by the indexing model from the issue text.