feat: scaffold OpenFeature (+ Microsoft.FeatureManagement provider) feature-flag support

[devantler]

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Part of devantler-tech/monorepo#2059 (feature-flag-first delivery portfolio-wide).

Problem

The .NET template ships no feature-flag scaffolding, so projects generated from it start without a way to land new features behind a flag.

Proposed direction

Scaffold the portable OpenFeature .NET SDK as the call-site API (matching the Go stack across the portfolio), with Microsoft.FeatureManagement as the provider via the official bridge — so generated apps keep MS FeatureManagement's IConfiguration/filter/variant machinery while call sites stay OpenFeature-portable:

• OpenFeature (v2.14.0, GA) as the app-facing API (IFeatureClient).
• Microsoft.FeatureManagement (4.5.0; v4 → IVariantFeatureManager) behind it via OpenFeature.Contrib.Provider.FeatureManagement (0.6.1), reading a FeatureManagement config section.
• One example flag (default-off) gating a sample code path + tests asserting both states; a short README/AGENTS.md ## Maintenance note on the convention + flag lifecycle (remove release flags after rollout).

Rough size

M — add the packages + a minimal wired example + both-state tests to the template.

References

OpenFeature .NET https://openfeature.dev/docs/reference/sdks/server/dotnet/ · bridge https://www.nuget.org/packages/openfeature.contrib.provider.featuremanagement · MS FeatureManagement https://learn.microsoft.com/en-us/azure/azure-app-configuration/feature-management-dotnet-reference

[devantler]

🤖 Generated by the Daily AI Assistant

Heads-up on the proposed direction: the Microsoft.FeatureManagement OpenFeature bridge (OpenFeature.Contrib.Provider.FeatureManagement) is preview-only on NuGet today (latest 0.1.2-preview; the 0.6.1 in the issue does not exist), and the template's toolchain policy avoids preview packages. So #263 scaffolds the GA OpenFeature in-memory provider instead (consistent with the Go template) and documents adopting the MS FeatureManagement bridge once it ships GA. Reopen the provider choice if you'd prefer the preview bridge.