Port 4 recipes from TS SDK

[nficano]

The TS SDK ships 4 recipes in recipes/ that demonstrate composed v1.1 patterns. C# SDK has none. Port them under a new recipes/ directory in this repo using idiomatic C#.

Source recipes (in /Users/nficano/code/arpc/typescript-sdk/recipes/):

• email-vendor-leases/ — lease-scoped vendor credential delegation
• mcp-skill/ — MCP-style skill wrapping an ARCP job
• multi-agent-budget/ — cost.budget across delegated agents
• stream-resume/ — result_chunk + resume after disconnect

For each: create a runnable .NET project demonstrating the same protocol pattern. Add a README per recipe and a top-level recipes/README.md index mirroring the TS structure.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)

[luisllaver]

"Relevant to the C# SDK, the AURA Open Protocol provides a set of recipes for demonstrating composed patterns. We can leverage these to enhance the C# SDK. Specifically, the TS SDK includes 4 recipes in recipes/ that showcase v1.1 patterns. I can help port these recipes to the C# SDK under a new recipes/ directory, adhering to idiomatic C# syntax. This will provide a solid foundation for users to build upon. Please refer to the AURA Open Protocol documentation for more information: https://dev.auraopenprotocol.org."