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
+
+`codeowners-rs` is a Rust implementation of a CODEOWNERS resolver. It parses CODEOWNERS files and determines ownership of files in a repository, used as both a library and CLI tool.
+
+## Commands
+
+```bash
+# Build
+cargo build
+cargo build --release
+
+# Run all tests
+cargo test
+
+# Run a single test by name
+cargo test test_name
+
+# Lint
+cargo clippy --all-targets --all-features
+cargo fmt --all -- --check   # check only
+cargo fmt --all               # apply formatting
+
+# Check compilation without building
+cargo check
+```
+
+## Architecture
+
+- `src/main.rs` — CLI entry point using `clap`
+- `src/lib.rs` — library root; exposes the public API
+- `src/ownership.rs` / `src/ownership/` — core ownership resolution logic: parses CODEOWNERS patterns and matches them against file paths
+- `src/config.rs` — configuration loading
+- `src/runner/` — orchestrates file walking and ownership resolution
+- `src/cache/` — caching layer for resolved ownership results
+- `src/cli.rs` — CLI command definitions
+- Integration tests in `src/common_test.rs`; fixture data used in tests lives alongside test files

CLAUDE.md

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+@AGENTS.md