Fix the .NET DevUI workflow sample

[ilia-sokolov]

Summary

• replace machine-specific Agent Framework project references with published packages
• load the documented .env file and require endpoint, model, and token configuration
• restore the intended FrontDesk-to-Concierge workflow order
• expose DevUI only in Development while retaining the OpenAI-compatible workflow endpoints
• align the launch profile, port, prerequisites, and GitHub Models documentation

Why

The .NET DevUI example could not build outside the original contributor's machine because it referenced absolute local source paths. It also hard-coded the model, reversed the documented workflow order, and documented configuration and ports that did not match the application.

The updated example is portable, provides actionable configuration errors, and can be run using a fine-grained GitHub token with Models: read permission.

Validation

• Release build completed with 0 warnings and 0 errors.
• /devui returned HTTP 200 in Development.
• /v1/entities exposed gh-model-workflow, FrontDesk, and Concierge.
• A deterministic local OpenAI-compatible stub completed a workflow request through both agents and returned the Concierge output.
• Missing configuration exits with an actionable error identifying the required variable and .env location.

Fixes #101