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
+
+PyWorkout is a minimal Python CLI application that guides users through workout sessions by muscle group. Users select a muscle group, then navigate exercises with commands (`list`, `start`, `next`, `skip`, `end`, `stats`, `video`, `help`, `license`, `quit`). The package is distributed on PyPI as `pyworkout` and as a Docker image on `ghcr.io`.
+
+## Common Commands
+
+```bash
+# Run the CLI locally
+python main.py
+
+# Install in editable mode for development
+pip install -e .
+
+# Run all tests
+pytest tests/ -v
+
+# Run a single test
+pytest tests/test_main.py::TestMuscleGroupSelection::test_abs_selection_by_number -v
+
+# Run tests with coverage report
+pytest tests/ --cov=. --cov-report=term-missing
+
+# Lint all Python files
+pylint $(git ls-files '*.py')
+
+# Build distributable packages
+python -m build
+```
+
+## Architecture
+
+The application is almost entirely contained in `main.py` as a single `workout()` function (~600 lines). This is intentional — the project is simple by design.
+
+**State is managed through module-level globals** set inside `workout()`: `select` (chosen muscle group index), `activity_num` (current exercise index), `start` (workout start time), `complete` (list of completed exercises), and `times` (per-exercise durations). Pylint complains about these; the file suppresses those warnings with inline `# pylint: disable=` comments, which is the established pattern for this codebase.
+
+**Workout data** is stored as parallel hardcoded lists at the top of `main.py` — one list of exercise names per muscle group (e.g., `abs`, `quads`) and a corresponding `_count` list of rep counts. Adding a new exercise means appending to both lists in sync.
+
+**Video playback** uses platform detection: `os.startfile()` on Windows, `subprocess` with `open` on macOS, and `xdg-open` on Linux. Video file paths are hardcoded absolute paths in `main.py` (lines 86–94) and must be updated by the user for their local setup.
+
+**`gui.py`** is an incomplete Tkinter GUI frontend — it is not integrated with `main.py` and is not functional. Do not treat it as a reference for current application behavior.
+
+## Testing Conventions
+
+Tests live in `tests/test_main.py` and use `pytest` with `pytest-mock`. All CLI I/O is tested by patching `builtins.input` and `builtins.print` via `@patch` decorators. Test classes map to functional areas:
+
+- `TestWorkoutData` — validates the parallel data list structure
+- `TestMuscleGroupSelection` — number and name-based group selection, invalid inputs
+- `TestWorkoutCommands` — each CLI command in isolation
+- `TestWorkoutFlow` — multi-step exercise sequences
+- `TestVideoFunctionality` — cross-platform video path dispatch
+- `TestIntegration` — full end-to-end scenarios
+
+Pytest configuration (in `setup.cfg`) auto-enables branch coverage and `--strict-markers`. Available markers: `@pytest.mark.slow`, `@pytest.mark.integration`.
+
+## CI/CD
+
+Five GitHub Actions workflows run automatically:
+
+| Workflow | Trigger | What it does |
+|---|---|---|
+| `tests.yml` | Push/PR to main or develop | pytest on Python 3.9–3.12, uploads coverage |
+| `pylint.yml` | Every push | Pylint check on Python 3.9 |
+| `docker-publish.yml` | Push to main, tags, PRs, daily | Build + push Docker image to ghcr.io |
+| `push-to-pypi.yml` | Release published | Build + publish to PyPI |
+| `codeql-analysis.yml` | Push, PRs, schedule | Security scanning |
+
+## Conventions
+
+- **Pylint suppressions**: Use inline `# pylint: disable=<rule>` for violations that are intentional given the single-function architecture. Do not add a `.pylintrc` file.
+- **Versioning**: Semantic versioning (e.g., `v1.2.0`). Version is defined in `setup.cfg` and `pyproject.toml`.
+- **Comments**: The codebase uses heavy inline comments per `CONTRIBUTING.md` — follow the same style when modifying `main.py`.
+- **No external runtime dependencies**: The CLI relies only on the Python standard library. Keep it that way.