Add AGENTS.md and CLAUDE.md for AI coding agent guidance

AGENTS.md

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+This file provides guidance to AI coding agents when working with code in this repository.
+
+## What this project is
+
+`danger-packwerk` is a [Danger](https://danger.systems/ruby/) plugin that integrates packwerk into pull request workflows. It runs `packwerk check` and posts inline PR comments for any new boundary violations introduced by a diff.
+
+## Commands
+
+```bash
+bundle install
+
+# Run all tests (RSpec)
+bundle exec rspec
+
+# Run a single spec file
+bundle exec rspec spec/path/to/spec.rb
+
+# Lint
+bundle exec rubocop
+bundle exec rubocop -a  # auto-correct
+
+# Type checking (Sorbet)
+bundle exec srb tc
+```
+
+## Architecture
+
+- `lib/danger_packwerk.rb` — Danger plugin entry point; defines the `packwerk` Danger DSL method
+- `lib/danger_packwerk/` — core logic: runs packwerk, diffs violations against the base branch, formats inline GitHub review comments
+- `spec/` — RSpec tests

CLAUDE.md

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+@AGENTS.md