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
+
+`code_ownership` is a Ruby gem that helps engineering teams declare and query ownership of code. It supports multiple ownership mechanisms: package-based ownership (via `package.yml`), team-based glob patterns, and file annotations.
+
+## Commands
+
+```bash
+bundle install
+
+# Run all tests (RSpec) — note: includes a native extension compile step
+bundle exec rake default   # compiles extension + runs specs
+
+# Run specs directly (after compiling)
+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/code_ownership.rb` — public API: `CodeOwnership.for_file`, `CodeOwnership.validate!`, `CodeOwnership.for_team`
+- `lib/code_ownership/` — mapper plugins (each ownership mechanism is a mapper), `Ownership` value object, configuration loading, and CLI
+- `spec/` — RSpec tests

CLAUDE.md

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+@AGENTS.md