chore: add AGENTS.md (doc layering)

AGENTS.md

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+# tradecloud-sdk-dotnet
+
+## Purpose
+
+.NET SDK for the TC1 API v2. One top-level dir per domain entity
+or connector, each containing per-operation .csproj samples
+(SendOrder, AttachOrderDocuments, FindIdentityByEmail, etc.).
+
+## Module / Stack
+
+C# / .NET 8.0. Single solution: `tradecloud-sdk-dotnet.sln`.
+No `Makefile`; refresh the solution with
+`dotnet sln add $(ls -r **/*.csproj)`.
+
+## Build
+
+`dotnet build` / `dotnet test` against the solution.
+
+## Entry Points
+
+- `tradecloud-sdk-dotnet.sln` -- solution
+- 17 top-level dirs (one per domain / connector / utility):
+  `api-connector/`, `authentication/`, `company/`, `conversation/`,
+  `forecast/`, `migration/`, `object-storage/`, `order/`,
+  `order-line-search/`, `order-search/`, `order-webhook-connector/`,
+  `sap-soap-connector/`, `sci-connector/`, `shipment/`,
+  `shipment-webhook-connector/`, `user/`, `workflow/`
+- `migration/` -- legacy SDK migration helpers
+- Per-operation `README.md` files inside each domain dir document
+  the sample's purpose
+
+## Notes
+
+- Mirrors the TC1 REST API surface; align changes with
+  `tradecloud-docs-api-v2`.