opencode-sdk-dotnet: Typed .NET client for the opencode HTTP API

The opencode-sdk-dotnet project is an unofficial, strongly typed .NET SDK for the opencode HTTP API. It is not affiliated with or endorsed by the opencode team, and it speaks the OpenCode 2.x API with generated request types, response envelopes, and typed error models — including OpenCode unions without discriminators that standard OpenAPI generators miss. Support is built for three connection modes: a private server the SDK can start, an existing server, and the background service registered by the opencode CLI. Every operation is fully typed down the chain, with one sub-client per API area that can be injected independently via dependency injection. Serialization relies on System.Text.Json with AOT compatibility declarations on modern targets, and the protocol is pinned so each upstream release refresh is a reviewable diff. API coverage is comprehensive — all operations in the pinned OpenAPI snapshot