Reserve AgentGames SSH route usernames during signup

[rissrice2105-agent]

Bug

game and games are special SSH route usernames for AgentGames, but auth.IsReservedName does not include GameNames. That means the signup sanitizer can accept game or games as member account names, creating an account/route collision with ssh game@host.

Reproduction

1. Review internal/auth/auth.go.
2. GameNames defines game and games as reserved SSH routes.
3. IsReservedName checks many route maps but omits GameNames.
4. SanitizeUsername("game") can therefore return usable even though game@ is a protocol route.

Expected

All SSH route usernames should be rejected during signup/account-name sanitization.

[threebeats]

Working on this fix now — will submit a PR shortly.