CLD-03: Add GitHub OAuth login for cloud instance

[Chris0Jeky]

Summary

Add GitHub OAuth as a login option for the hosted cloud instance. Developer audience makes GitHub the natural first OAuth provider.

Acceptance Criteria

• Users can sign in with GitHub OAuth on the cloud instance
• GitHub user profile (name, email, avatar) used for account creation
• Existing username/password login still works (for local instances)
• OAuth config is environment-gated (only active when GitHub client ID/secret are configured)
• New users created via OAuth get a default workspace

Implementation Notes

• ASP.NET Core has built-in GitHub OAuth middleware
• Store GitHub user ID as external login link
• Consider: should local instances also support OAuth? (probably not initially)
• Google OAuth can be added as a follow-on

Parent

#537