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
+
+`singed` makes it easy to get a flamegraph anywhere in a Ruby codebase. It wraps profiling with [stackprof](https://github.com/tmm1/stackprof) or [rbspy](https://github.com/rbspy/rbspy) and launches [speedscope](https://github.com/jlfwong/speedscope) to view results.
+
+## 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
+```
+
+## Architecture
+
+- `lib/singed.rb` — main entry point; provides `Singed.flamegraph` block helper
+- `lib/singed/` — core classes: flamegraph output handling, stackprof/rbspy integrations, speedscope launcher
+- `spec/` — RSpec tests

CLAUDE.md

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+@AGENTS.md