feat: Add C# SDK

[lox]

Summary

Adds a C# SDK for defining Buildkite pipelines programmatically. Targets .NET 8.0 and .NET 9.0.

Usage

using Buildkite.Sdk;
using Buildkite.Sdk.Schema;

var pipeline = new Pipeline();
pipeline.AddStep(new CommandStep
{
    Label = ":hammer: Build",
    Command = "dotnet build"
});

Console.WriteLine(pipeline.ToYaml());

What's included

• Pipeline builder with JSON/YAML serialization
• Environment helper for type-safe access to Buildkite env vars
• Step types: CommandStep, BlockStep, InputStep, WaitStep, TriggerStep, GroupStep
• 31 passing tests
• CI/CD integration with NuGet publishing
• Example app in apps/csharp/

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 27048cd887

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[lox]

@codex review again please!

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 6c48b6ad83

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[lox]

@codex this has been substantially refactored to use union types better, can you dig in and deeply review please?

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: bea510a16c

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[HugeIRL]

@lox had a review here and things look great! All tests pass locally and with the example c# program.

Just a few things you may want to consider but shouldn't be merge blocking:

1. No test coverage for Plugins/Secrets: There are no tests validating that plugins or secrets serialize correctly.
   Here's a rough example to consider:

  [Test]
  public void CommandStep_WithPlugins_SerializesCorrectly()
  {
      var step = new CommandStep
      {
          Plugins = new object[] {
              "docker#v5.0.0",
              new Dictionary<string, object> {
                  ["ecr#v2.0.0"] = new { login = true }
              }
          }
      };
      // Assert serialization matches expected format
  }

2. Documentation examples use C# 12 syntax: The examples use collection expressions which require C# 12. If the SDK targets older versions, users might find the examples confusing.

3. Should there be runtime validation? Since Plugins accepts object?, invalid types will only fail at serialization time. Is that acceptable, or should there be validation?

Here's an example:

  // All of these may compile successfully, even though some are wrong:
  new CommandStep
  {
      Plugins = "docker#v5.0.0",  // ✅ Valid
      Plugins = new object[] { "docker#v5.0.0" },  // ✅ Valid
      Plugins = new Dictionary<string, object> { ["ecr"] = new { login = true } },  // ✅ Valid
      Plugins = 42,  // ❌ Wrong, but compiles!
      Plugins = new { foo = "bar" },  // ❌ Wrong, but compiles!
      Plugins = new List<int> { 1, 2, 3 },  // ❌ Wrong, but compiles!
  }