Add AGENTS.md file

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to AI agents
+
+## Project Overview
+
+ChefSpec is a unit testing framework for Chef cookbooks. It runs cookbooks locally with resource actions disabled, enabling fast feedback without requiring virtual machines or cloud servers.
+
+## Common Commands
+
+```bash
+# Run all tests (unit + acceptance)
+bundle exec rake
+
+# Run only unit tests
+bundle exec rake unit
+
+# Run only acceptance tests
+bundle exec rake acceptance
+
+# Run a single acceptance test example
+bundle exec rake acceptance:<example_name>  # e.g., acceptance:custom_resource
+
+# Run a specific unit test file
+bundle exec rspec spec/unit/<file>_spec.rb
+
+# Lint with Cookstyle (ChefStyle)
+bundle exec rake style
+```
+
+## Architecture
+
+### Runners
+
+- **SoloRunner** (`lib/chefspec/solo_runner.rb`): The primary runner that simulates a Chef Solo run. Creates a `Chef::Client`, loads Fauxhai data, and converges recipes without actually executing resource actions.
+- **ServerRunner** (`lib/chefspec/server_runner.rb`): Extends SoloRunner to simulate a Chef Server environment using chef-zero.
+- **Runner** (`lib/chefspec/runner.rb`): Alias for SoloRunner for backward compatibility.
+
+### API Layer
+
+`lib/chefspec/api/` contains modules that provide the RSpec DSL:
+
+- **Core** (`api/core.rb`): Provides `platform`, `recipe`, `step_into`, `*_attributes`, `chefspec_options` helpers. Sets up `chef_run`, `chef_runner`, `chef_node` let variables.
+- **Stubs** (`api/stubs.rb`): `stub_command`, `stub_search`, `stub_data_bag`, `stub_data_bag_item`
+- **StubsFor** (`api/stubs_for.rb`): `stubs_for_resource`, `stubs_for_provider`, `stubs_for_current_value`
+- **RenderFile** (`api/render_file.rb`): `render_file` matcher
+- **Notifications** (`api/notifications.rb`): `notify` matcher
+- **Subscriptions** (`api/subscriptions.rb`): `subscribe_to` matcher
+
+### Matchers
+
+`lib/chefspec/matchers/` contains RSpec matchers:
+
+- **ResourceMatcher**: Dynamically generated `action_resource(name)` matchers (e.g., `create_file`, `install_package`)
+- **RenderFileMatcher**: For testing file content from template/file/cookbook_file resources
+- **NotificationsMatcher** / **SubscribesMatcher**: For testing resource notifications
+
+### Extensions
+
+`lib/chefspec/extensions/` patches Chef classes to:
+- Disable actual resource execution (actions become no-ops)
+- Track performed actions for matcher verification
+- Handle shell_out stubbing
+- Support `step_into` for custom resources
+
+### Stubs System
+
+`lib/chefspec/stubs/` provides registries and stub objects for:
+- Shell commands (guard clauses)
+- Search queries
+- Data bags and data bag items
+
+## Test Structure
+
+- **Unit tests** (`spec/unit/`): Test ChefSpec's own internals
+- **Acceptance tests** (`examples/`): Each subdirectory is a mini-cookbook testing a specific feature. Run via `rake acceptance:<dirname>`
+
+## Key Patterns
+
+Resource matchers follow `action_resource(name)` pattern:
+```ruby
+it { is_expected.to create_file('/etc/config') }
+it { is_expected.to install_package('nginx').with(version: '1.0') }
+```
+
+Testing custom resources requires `step_into`:
+```ruby
+step_into :my_custom_resource
+# or
+ChefSpec::SoloRunner.new(step_into: ['my_custom_resource'])
+```