CSHARP-5943: Initial small AGENTS.md file for the C# driver

See notes in DRIVERS-3428.

Author: ajcvickers

AGENTS.md

@@ -0,0 +1,47 @@
+# AGENTS.md - CSharpDriver
+
+## Overview
+The C# driver for MongoDB.
+
+## Tech Stack
+- .NET library projects producing NuGet packages
+- Multi-targeted to various .NET versions from .NET Framework 4.7.2 up
+- xUnit + FluentAssertions for testing
+
+## Project Structure
+- `src/MongoDB.Bson/` - BSON for MongoDB
+- `src/MongoDB.Driver/` - C# driver for MongoDB
+- `src/MongoDB.Driver.Encryption/` - Client encryption (CSFLE with KMS).
+- `src/MongoDB.Driver.Authentication.AWS/` - AWS IAM authentication
+- `tests/MongoDB.Driver.Tests/` - Main C# driver tests
+- `tests/MongoDB.Bson.Tests/` - BSON handling tests
+- `tests/*/TestHelpers` - Common test utilities
+- `tests/*` - Specialized tests; less common
+- `tests/MongoDB.Driver.Tests/Specifications/` are JSON-driven tests using a common runner.
+
+## Commands
+- Build: `dotnet build CSharpDriver.sln`
+- Run all tests: `dotnet test tests/MongoDB.Driver.Tests/MongoDB.Driver.Tests.csproj -f net10.0`
+- Run a single test class: `dotnet test tests/MongoDB.Driver.Tests/MongoDB.Driver.Tests.csproj -f net10.0 --filter "FullyQualifiedName~ClassName"`
+
+A MongoDB connection is always available locally, so "integration" tests can be run as well as unit tests. Some test suites also require additional environment variables — if you need to run those tests and the variables are not set, stop and tell the user which variables are needed rather than working around it.
+
+| Feature area | Required environment variables |
+|---|---|
+| Atlas Search | `ATLAS_SEARCH_TESTS_ENABLED`, `ATLAS_SEARCH_URI` |
+| Atlas Search index helpers | `ATLAS_SEARCH_INDEX_HELPERS_TESTS_ENABLED`, `ATLAS_SEARCH_URI` |
+| CSFLE / auto-encryption | `CRYPT_SHARED_LIB_PATH` |
+| CSFLE with KMS mock servers | `KMS_MOCK_SERVERS_ENABLED` |
+| CSFLE with AWS KMS | `CSFLE_AWS_TEMPORARY_CREDS_ENABLED` |
+| CSFLE with Azure KMS | `CSFLE_AZURE_KMS_TESTS_ENABLED` |
+| CSFLE with GCP KMS | `CSFLE_GCP_KMS_TESTS_ENABLED` |
+| AWS authentication | `AWS_TESTS_ENABLED` |
+| GSSAPI / Kerberos | `GSSAPI_TESTS_ENABLED`, `AUTH_HOST`, `AUTH_GSSAPI` |
+| OIDC authentication | `OIDC_ENV` |
+| X.509 authentication | `MONGO_X509_CLIENT_CERTIFICATE_PATH`, `MONGO_X509_CLIENT_CERTIFICATE_PASSWORD` |
+| PLAIN authentication | `PLAIN_AUTH_TESTS_ENABLED` |
+| SOCKS5 proxy | `SOCKS5_PROXY_SERVERS_ENABLED` |
+
+## Commit and PR Conventions
+
+- Commit and PR messages start with a JIRA number: `CSHARP-1234: Description`