Add AGENTS.md and CLAUDE.md for AI coding agent guidance (#10)

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

* Fix inaccuracies in AGENTS.md

- Remove redundant heading that repeats the filename
- Fix incorrect spec/fixtures/ references where that directory doesn't exist
- Fix Claude-specific intro text to be agent-agnostic (pks only)

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 +1,23 @@
+This file provides guidance to AI coding agents when working with code in this repository.
+
+## What this project is
+
+`code_manifest` is a Ruby gem that fetches files by glob patterns and generates a digest of those files. It is used to produce stable manifests of file sets for caching and change-detection purposes.
+
+## Commands
+
+```bash
+bundle install
+
+# Run all tests (RSpec)
+bundle exec rspec
+
+# Run a single spec file
+bundle exec rspec spec/path/to/spec.rb
+```
+
+## Architecture
+
+- `lib/code_manifest.rb` — public API; accepts glob patterns and returns matched files plus a digest
+- `lib/code_manifest/` — internal helpers for glob resolution and hashing
+- `spec/` — RSpec tests; `spec/fixtures/` holds sample file trees

CLAUDE.md

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