Add C# / .NET skills + backport OAuth improvements to JS

[ericsche]

Summary

Adds C# / .NET equivalents of the three JS agent-plugin skills, plus backports improvements discovered during development to the JS versions.

New .NET Skills (3)

Skill | Purpose -- | -- agents-sdk-dotnet | Main SDK reference — NuGet packages, Program.cs structure, appsettings.json config, AgentApplication routing, auth/OAuth (UserAuthorization), cards, streaming, proactive messaging, state, OpenTelemetry, common mistakes agents-sdk-dotnet-debugging | 6-step debugging checklist: build → start → reachability → config validation → Azure AD credentials → Agents Playground azure-agents-sdk-provision-dotnet | Azure provisioning — all 3 auth types (UserManagedIdentity, FederatedCredentials, ClientSecret), Bot resource creation via az rest, appsettings.json output, Key Vault / user-secrets patterns

Each skill includes reference files mirroring the JS structure:

• references/cards.md — Adaptive Cards, Hero/Thumbnail cards, Action Execute, Search handlers
• references/streaming.md — basic streaming, Azure OpenAI streaming, final card messages
• references/oauth-setup.md — ClientSecret (AadV2), FIC (AadV2WithFic), Teams SSO, API permissions

JS Backported Improvements

• agents-sdk-typescript/references/cards.md — Added Adaptive Cards resource table pointing to the new official site (adaptivecards.microsoft.com), replacing the obsolete adaptivecards.io
• azure-agents-sdk-provision/references/oauth-setup.md — Added:
  • Regional OAuth redirect URL table (Global, Europe, US, India, Gov, 21Vianet)
  • Recommended auth types per environment (ClientSecret for dev, MSI for prod)
  • Link to FIC user authorization docs on Microsoft Learn

Other Changes

• .claude-plugin/plugin.json added for Claude compatibility
• plugin.json version bumped from 1.0.0 → 1.1.0

[tracyboehrer]

@ericsche Thanks for this. This provides a good basis for building out these type skills. Much appreciated.