Add AGENTS.md for AI guidance and create CLAUDE.md referencing it

Author: seanthegeek

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to AI agents when working with code in this repository.
+
+## Project Overview
+
+checkdmarc is a Python library and CLI tool for validating email security DNS records (SPF, DMARC, BIMI, MTA-STS, SMTP TLS Reporting, MX/STARTTLS, DNSSEC, SOA). Published on PyPI as `checkdmarc`.
+
+## Common Commands
+
+```bash
+# Run tests with coverage
+coverage run tests.py
+
+# Lint and format
+ruff check --show-fixes
+ruff format .
+
+# Build package
+hatch build
+
+# Build docs
+cd docs && make html
+
+# Full build (format + docs + package)
+./build.sh
+```
+
+Tests use `unittest.TestCase` in a single `tests.py` file. Run a single test with:
+
+```bash
+python -m pytest tests.py -k "test_name"
+```
+
+Some tests require network access and are skipped when `GITHUB_ACTIONS` env var is set.
+
+## Architecture
+
+**Entry point:** `checkdmarc/__init__.py` — `check_domains()` orchestrates all checks, returning `DomainCheckResult` TypedDict(s).
+
+**Modules** (each has a primary `check_*()` function):
+
+- `spf.py` — SPF record parsing, DNS lookup counting
+- `dmarc.py` — DMARC/DMARCbis record parsing with DNS tree walk algorithm
+- `bimi.py` — BIMI record and certificate validation
+- `mta_sts.py` — MTA-STS policy fetching and validation
+- `smtp_tls_reporting.py` — TLSRPT record validation
+- `smtp.py` — MX record lookup and STARTTLS testing
+- `dnssec.py` — DNSSEC validation
+- `soa.py` — SOA record parsing
+- `utils.py` — DNS helpers, exception classes, domain normalization
+
+**CLI:** `_cli.py` (entry point: `checkdmarc._cli:_main`)
+
+**Constants/version:** `_constants.py`
+
+**Output:** `results_to_json()`, `results_to_csv()`, `output_to_file()` in `__init__.py`.
+
+## Key Dependencies
+
+- `dnspython` for DNS queries
+- `pyleri` for grammar parsing
+- `publicsuffixlist` for base domain extraction
+- `cryptography`/`pyopenssl`/`pem` for certificate handling
+- `expiringdict` for DNS result caching
+
+## Code Style
+
+- Formatter/linter: **Ruff**
+- Type annotations use `TypedDict` for structured results
+- Python >=3.10 required
+- Build backend: **hatchling**

CLAUDE.md

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+# CLAUDE.md
+
+@AGENTS.md