Add AGENTS.md for Cursor Cloud development environment setup

[jamesra]

Summary

Adds AGENTS.md with Cursor Cloud-specific development environment instructions for the Viking Identity Server subsystem.

What's included

• Complete setup instructions for the .NET 9.0 IdentityServer solution on Linux
• SQL Server Docker container configuration
• Service startup commands (IdentityServerStandalone, WebApi, WebManagement)
• Build, test, and lint commands
• Non-obvious gotchas documented (SSL fallback, EF migration vs EnsureCreated test issue, ConfigurationSubstitutor usage, Duende dev license)

Evidence of working environment

Identity Server home page

Viking Identity Server Home Page

OIDC Discovery Endpoint

OIDC Discovery Endpoint

WebApi Swagger UI

WebApi Swagger UI

OAuth2 Token Flow

token_test_output.log

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