docs: add CLAUDE.md with codebase guidance for AI assistants

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
+
+ProgramVer is a Python/tkinter GUI app that replicates Microsoft's `winver` — it displays a customizable window with program version info, copyright notices, and buttons to open a License or EULA file in a secondary window. It is published to PyPI and is designed to be forked and customized per-program.
+
+## Commands
+
+### Install dependencies
+```bash
+pip install -r requirements.txt
+```
+
+### Run all tests
+```bash
+# Linux (requires xvfb for headless tkinter)
+xvfb-run -a python -m pytest tests/ -v
+
+# Windows / macOS
+python -m pytest tests/ -v
+```
+
+### Run a single test
+```bash
+# Linux
+xvfb-run -a python -m pytest tests/test_main.py::TestProgramVer::test_name -v
+
+# Windows / macOS
+python -m pytest tests/test_main.py::TestProgramVer::test_name -v
+```
+
+### Run tests with coverage
+```bash
+xvfb-run -a python -m pytest tests/ --cov=. --cov-report=term-missing
+```
+
+### Lint
+```bash
+pylint $(git ls-files '*.py')
+```
+
+## Architecture
+
+All application logic lives in a single module: **`main.py`**. It exposes four functions:
+
+- `get_resource_path(filename)` — resolves paths relative to the module file (needed for PyPI installs where the CWD may differ from the package location).
+- `ProgramVer()` — builds and runs the main tkinter window: logo images, version/copyright labels, and two buttons.
+- `openLicense()` — opens `LICENSE.txt` in a new `Tk()` window.
+- `openEULA()` — opens `EULA.txt` in a new `Tk()` window.
+
+`__main__.py` is the entry point; it just calls `ProgramVer()`. `setup.cfg` / `pyproject.toml` register the `programver` console script pointing at `main:ProgramVer`.
+
+**Customization intent:** The strings inside `ProgramVer()` (window title, version label, trademark text, license blurb) and the image files in `imgs/` are expected to be replaced when the project is forked. `LICENSE.txt` and `EULA.txt` in the repo root are the files opened at runtime.
+
+## Testing
+
+Tests are in `tests/test_main.py` using `unittest.TestCase`. All tkinter calls are mocked with `unittest.mock.patch` so tests run headlessly. The `# pylint: disable=import-error, invalid-name` comments at the top of both `main.py` and `test_main.py` are intentional — do not remove them.
+
+CI runs the full matrix: Ubuntu, Windows, macOS × Python 3.9–3.12. Coverage target is 100% for `main.py`.
+
+## Coding Conventions
+
+- 4-space indentation (no tabs).
+- Semantic Versioning for releases.
+- Version number appears in `main.py` (the `info` label), `pyproject.toml`, `setup.cfg`, and `setup.py` — update all four on a version bump.