Add initial stub of agents file

AGENTS.md

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+# AI Assistant Guide for docs-builder
+
+This file contains instructions and guidance for AIs when working with the docs-builder repository.
+
+## Repository overview
+
+This is Elastic's distributed documentation tooling system built on .NET 9, consisting of:
+
+- **docs-builder**: CLI tool for building single documentation sets
+- **docs-assembler**: CLI tool for assembling multiple doc sets
+- Written in C# and F# with extensive Markdown processing capabilities
+
+## Essential commands
+
+### Development
+```bash
+# Build the project
+dotnet build
+
+# Simulates a full release -- run this to confirm the absence of build errors
+dotnet run --project build -c release
+
+# Runs all tests
+dotnet test
+
+# Run docs-builder locally
+./build.sh run -- serve
+
+# Clean build artifacts
+dotnet clean
+```
+
+### Linting and Code Quality
+
+```bash
+# Format code. Always run this when output contains formatting errors.
+dotnet format
+
+# Run specific test project
+dotnet test tests/Elastic.Markdown.Tests/
+
+# Run tests with verbose output
+dotnet test --logger "console;verbosity=detailed"
+```
+
+## Key architecture Points
+
+### Main Projects
+
+- `src/Elastic.Markdown/` - Core Markdown processing engine
+- `src/tooling/docs-builder/` - Main CLI application
+- `src/tooling/docs-assembler/` - Assembly tool
+- `src/Elastic.Documentation.Site/` - Web rendering components
+
+### Testing Structure
+
+- `tests/` - C# unit tests
+- `tests/authoring/` - F# authoring tests
+- `tests-integration/` - Integration tests
+
+### Configuration
+
+- `config/` - YAML configuration files
+- `Directory.Build.props` - MSBuild properties
+- `Directory.Packages.props` - Centralized package management
+
+## Development guidelines
+
+### Adding New Features
+
+1. **Markdown Extensions**: Add to `src/Elastic.Markdown/Myst/`
+2. **CLI Commands**: Extend `src/tooling/docs-builder/Cli/` or `docs-assembler/Cli/`
+3. **Web Components**: Add to `src/Elastic.Documentation.Site/`
+4. **Configuration**: Modify `src/Elastic.Documentation.Configuration/`
+
+### Code style
+
+- Follow existing C# and F# conventions in the codebase
+- ...
+
+### Testing requirements
+
+- Add unit tests for new functionality
+- Use F# for authoring/documentation-specific tests
+- ...
+
+### Common patterns
+
+- ...
+
+## Documentation
+
+The repository is self-documenting:
+
+- `/docs/` contains comprehensive documentation
+
+You MUST update the documentation when there are changes in the markdown syntax or rendering behaviour.
+
+## Useful file locations
+
+- Entry points: `src/tooling/docs-builder/Program.cs`, `src/tooling/docs-assembler/Program.cs`
+- Markdown processing: `src/Elastic.Markdown/Myst/`
+- Web assets: `src/Elastic.Documentation.Site/Assets/`
+- Configuration schemas: `src/Elastic.Documentation.Configuration/`
+- Test helpers: `tests/Elastic.Markdown.Tests/TestHelpers.cs`