docs: add CLAUDE.md for Claude Code guidance

Provides architecture overview, common commands, and testing info
for AI-assisted development.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+OCRmyPDF adds an OCR text layer to scanned PDF files, making them searchable. It uses Tesseract OCR and Ghostscript as external dependencies.
+
+## Common Commands
+
+```bash
+# Run all tests (uses pytest-xdist for parallel execution)
+pytest
+
+# Run a single test file
+pytest tests/test_main.py
+
+# Run a specific test
+pytest tests/test_main.py::test_function_name
+
+# Run tests with coverage
+pytest --cov=src/ocrmypdf --cov-report=html
+
+# Run slow tests (disabled by default)
+pytest --runslow
+
+# Lint and format
+ruff check src/
+ruff format src/
+
+# Type checking
+mypy src/ocrmypdf
+```
+
+## Architecture
+
+### Entry Points
+- **CLI**: `src/ocrmypdf/__main__.py` → `src/ocrmypdf/cli.py` parses arguments
+- **Python API**: `src/ocrmypdf/api.py` provides `ocr()` function for programmatic use
+
+### Core Pipeline
+The OCR pipeline is in `src/ocrmypdf/_pipeline.py` and `src/ocrmypdf/_pipelines/`. Processing flow:
+1. Input validation and triage (PDF vs image)
+2. PDF info extraction (`src/ocrmypdf/pdfinfo/`)
+3. Page-by-page OCR processing (parallelized)
+4. PDF/A generation and optimization
+
+### Options Model
+`src/ocrmypdf/_options.py` contains `OCROptions`, a Pydantic model that validates all CLI and API options. Options validation happens in `src/ocrmypdf/_validation.py` with cross-cutting validation in `src/ocrmypdf/_validation_coordinator.py`.
+
+### Plugin System
+OCRmyPDF uses `pluggy` for extensibility. Key files:
+- `src/ocrmypdf/pluginspec.py`: Defines all hook specifications
+- `src/ocrmypdf/builtin_plugins/`: Default implementations
+  - `tesseract_ocr.py`: Tesseract OCR engine
+  - `ghostscript.py`: PDF rasterization and PDF/A generation
+  - `optimize.py`: PDF optimization
+
+Plugins can replace the OCR engine, add CLI arguments, or modify image processing.
+
+### External Tool Wrappers
+`src/ocrmypdf/_exec/` contains wrappers for external tools:
+- `ghostscript.py`: PDF rasterization, PDF/A conversion
+- `tesseract.py`: OCR engine interface
+- `unpaper.py`: Image preprocessing (deskew, clean)
+- `jbig2enc.py`, `pngquant.py`: Image optimization
+
+### Job Context
+- `PdfContext`: Document-level context passed through pipeline
+- `PageContext`: Per-page context for parallel processing
+
+## Testing
+
+Tests are in `tests/` with fixtures defined in `tests/conftest.py`. Key fixtures:
+- `resources`: Path to test PDF/image files in `tests/resources/`
+- `outpdf`: Temporary output PDF path
+- `check_ocrmypdf()`: Run OCR and assert valid output
+- `run_ocrmypdf_api()`: Run via API, returns ExitCode
+- `run_ocrmypdf()`: Run as subprocess
+
+## External Dependencies
+
+Requires system packages: Tesseract OCR, Ghostscript. Optional: unpaper, jbig2enc, pngquant.
+
+## License
+
+MPL-2.0 for core code. Tests and docs use CC-BY-SA-4.0.