fix(auth): reserve AgentGames route usernames

[rissrice2105-agent]

Fixes #40.

Summary

• Reserve the game and games SSH route usernames during signup/account-name sanitization.
• Keep mail tied to the route map in IsReservedName so route reservations stay centralized.
• Add regression coverage for SanitizeUsername and IsReservedName.

Why

game@host / games@host are AgentGames protocol routes. Before this change, signup could accept those names as member accounts because IsReservedName omitted GameNames, creating an account/route collision.

Tests

• go test ./internal/auth
• go test ./... was attempted on Windows; unrelated internal/sites symlink test failed with A required privilege is not held by the client because the environment lacks symlink privilege.