Add AGENTS.md and CLAUDE.md for AI coding agent guidance (#177)

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

* Fix inaccuracies in AGENTS.md

- Remove redundant `# AGENTS.md` heading (filename already identifies it)
- Remove nonexistent `Formatter` and `Pack` classes from architecture description
  (`Pack` comes from the `packs-specification` gem; there is no `Formatter` in lib/packs/)
- Fix `spec/fixtures/` reference — this directory doesn't exist; tests use
  temp directories via `packs/rspec/support`
- Add `lib/packs/private/` entry explaining the internal implementation layer
- Note that `bin/packs` runs interactive mode when called with no arguments
- List additional CLI commands (`validate`, `check`) for completeness

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: dduugg

AGENTS.md

@@ -0,0 +1,32 @@
+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

@@ -0,0 +1 @@
+@AGENTS.md