Migrate to AGENTS.md following https://agents.md/ standard

- Adopt AGENTS.md format instead of custom AGENT.md
- Maintain all essential setup, testing, and development information
- Follow open standard for AI agent guidance used by 60k+ projects
- Ensure compatibility with multiple AI coding agents

*Migration performed using Big Pickle/OpenCode Zen*

Author: highb

AGENT.md

@@ -0,0 +1,121 @@
+# AGENTS.md
+
+PathSpec Ruby - .gitignore-style pattern matching in Ruby
+
+## Project overview
+
+Ruby gem implementing .gitignore-style pattern matching with both library API and CLI tool. Supports Ruby 3.2-4.0.1 with comprehensive test coverage and multi-Ruby CI.
+
+## Setup commands
+
+```bash
+# Install mise (Ruby version manager)
+brew install mise  # macOS
+# Other platforms: https://mise.jdx.dev/getting-started.html
+
+# Activate mise
+eval "$(mise activate zsh)"  # or bash, fish, etc.
+
+# Install Ruby and bundler versions
+mise install
+
+# Install dependencies
+mise run install
+# or: bundle install
+```
+
+## Testing
+
+```bash
+# Run all tests (lint, unit, integration, docs)
+mise run test
+# or: bundle exec rake
+
+# Unit tests only
+mise run test:unit
+# or: bundle exec rake spec
+
+# Integration tests (CLI) only
+mise run test:integration
+# or: bundle exec rake spec_integration
+
+# All specs (unit + integration)
+mise run test:all
+# or: bundle exec rake spec_all
+
+# Cross-Ruby matrix testing via Docker
+mise run test:matrix
+# or: bundle exec rake test_matrix
+```
+
+## Code style
+
+- Use RuboCop 1.63.5 for linting
+- Method length limit: 69 lines
+- Use single quotes for strings without interpolation
+- Use `%w[]` for word arrays
+- Auto-fix with: `bundle exec rubocop --autocorrect`
+- For large data arrays: add `# rubocop:disable Metrics/MethodLength`
+
+## Build and release
+
+```bash
+# Build gem
+mise run build
+# or: gem build pathspec.gemspec
+
+# Generate documentation
+bundle exec rake docs
+
+# Development install
+rake install
+```
+
+## Project structure
+
+```
+pathspec-ruby/
+├── lib/pathspec/           # Core library
+│   ├── pathspec.rb       # Main PathSpec class
+│   └── patterns/         # Pattern implementations
+├── bin/pathspec-rb       # CLI executable
+├── spec/
+│   ├── unit/             # Library tests
+│   └── integration/      # CLI tests
+├── benchmarks/           # Performance tests
+├── docs/                # Documentation source
+├── Rakefile            # Build tasks
+├── .tool-versions       # Ruby/bundler versions
+└── pathspec.gemspec     # Gem spec
+```
+
+## Development workflow
+
+1. Make changes to `lib/` code
+2. Add/update tests in `spec/unit/` for library changes
+3. Add/update tests in `spec/integration/` for CLI changes
+4. Run `mise run test` - must pass before committing
+5. Fix any RuboCop offenses
+6. Test cross-Ruby with `mise run test:matrix`
+7. Commit with descriptive messages
+
+## CLI tool usage
+
+```bash
+bundle exec pathspec-rb -f .gitignore match "file.swp"
+bundle exec pathspec-rb -f .gitignore specs_match "file.swp"
+bundle exec pathspec-rb -f .gitignore tree ./src
+```
+
+## Common issues
+
+**Bundler conflicts**: Always use `mise run install` to ensure correct versions from `.tool-versions`
+
+**RuboCop failures**: Auto-fix with `bundle exec rubocop --autocorrect`. Method length is common issue - extract large data arrays to separate methods with rubocop:disable comments
+
+**CI failures**: Check GitHub Actions. RuboCop and integration test failures are most common causes
+
+## Dependencies
+
+- **Runtime**: None (pure Ruby)
+- **Development**: rspec (~> 3.10), rubocop (~> 1.63.0), fakefs (~> 2.5), kramdown (~> 2.3), benchmark-ips (~> 2.0)