Add repository guidance doc

Author: HavenDV

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to Codex (Codex.ai/code) when working with code in this repository.
+
+## Project Overview
+
+C# SDK for the [AI21](https://ai21.com/) AI platform, auto-generated from the AI21 OpenAPI specification using [AutoSDK](https://github.com/HavenDV/AutoSDK). Published as a NuGet package under the `AI21` package name.
+
+## Build Commands
+
+```bash
+# Build the solution
+dotnet build AI21.slnx
+
+# Build for release (also produces NuGet package)
+dotnet build AI21.slnx -c Release
+
+# Run integration tests (requires AI21_API_KEY env var)
+dotnet test src/tests/AI21.IntegrationTests/AI21.IntegrationTests.csproj
+
+# Regenerate SDK from OpenAPI spec
+cd src/libs/AI21 && ./generate.sh
+```
+
+## Architecture
+
+### Code Generation Pipeline
+
+The SDK code is **entirely auto-generated** -- do not manually edit files in `src/libs/AI21/Generated/`.
+
+1. `src/libs/AI21/openapi.yaml` -- the AI21 OpenAPI spec (fetched from `https://api.ai21.com/openapi.json`)
+2. `src/helpers/FixOpenApiSpec/` -- converts OpenAPI 3.1 to 3.0 format for compatibility
+3. `src/libs/AI21/generate.sh` -- orchestrates: download spec, fix spec, run AutoSDK CLI, output to `Generated/`
+4. CI auto-updates the spec and creates PRs if changes are detected
+
+### Project Layout
+
+| Project | Purpose |
+|---------|---------|
+| `src/libs/AI21/` | Main SDK library (`Ai21Api`) |
+| `src/tests/AI21.IntegrationTests/` | Integration tests against real AI21 API |
+| `src/helpers/FixOpenApiSpec/` | OpenAPI spec fixer tool |
+| `src/helpers/GenerateDocs/` | Documentation generator from integration tests |
+| `src/helpers/TrimmingHelper/` | NativeAOT/trimming compatibility validator |
+
+### Build Configuration
+
+- **Target:** `net10.0` (single target)
+- **Language:** C# preview with nullable reference types
+- **Signing:** Strong-named assemblies via `src/key.snk`
+- **Versioning:** Semantic versioning from git tags (`v` prefix) via MinVer
+- **Analysis:** All .NET analyzers enabled, AOT/trimming compatibility enforced
+- **Testing:** MSTest + FluentAssertions
+
+### Key Conventions
+
+- No hand-written extension files -- the SDK is purely generated code
+- The client class is named `Ai21Api`
+- The namespace is `AI21`
+
+### CI/CD
+
+- Uses shared workflows from `HavenDV/workflows` repo
+- Dependabot updates NuGet packages weekly (auto-merged)
+- Documentation deployed to GitHub Pages via MkDocs Material