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
+
+`packs` is a Ruby gem that provides the development CLI for the packs packaging system — a way to modularize Ruby applications. It provides `bin/packs` commands to create packs, move files between packs, add dependencies, and manage pack configuration.
+
+## 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/packs.rb` — public API entry point
+- `lib/packs/` — core library: `Configuration`, `CLI` (Thor-based command definitions), post-processors (`RubocopPostProcessor`, `CodeOwnershipPostProcessor`, `UpdateReferencesPostProcessor`), and the `UserEventLogger` interface
+- `lib/packs/private/` — internal implementation; `interactive_cli/` contains the interactive TUI mode
+- `bin/packs` — CLI executable; runs the interactive mode when called with no arguments, otherwise dispatches to `Packs::CLI` commands like `create`, `add_dependency`, `move`, `validate`, `check`
+- `spec/` — RSpec tests; tests use temporary directories created via `packs/rspec/support` rather than static fixtures

CLAUDE.md

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+@AGENTS.md