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
+
+`parse_packwerk` is a low-dependency Ruby gem for parsing and writing the YAML files that packwerk uses: `package.yml` (package config) and `package_todo.yml` (recorded violations).
+
+## 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/parse_packwerk.rb` — entry point; `ParsePackwerk.all` returns all packages
+- `lib/parse_packwerk/` — `Package` (parses `package.yml`), `PackageTodo` (parses `package_todo.yml`), and YAML write-back helpers that preserve comments and ordering
+- `spec/` — RSpec tests

CLAUDE.md

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+@AGENTS.md