docs: add CLAUDE.md for project guidance and setup

Add CLAUDE.md to provide detailed guidance for using the 
repository. Include project overview, build commands, release 
process, and conventional commits. This enhances documentation 
and aids developers in understanding and contributing to the 
project efficiently.

CLAUDE.md

@@ -0,0 +1,30 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This repository builds a Docker image for running pre-commit hooks in CI/CD pipelines. The image is based on Go and includes Python (via pipx), Ruby, and common development tools like goimports and go-pre-commit.
+
+## Build Commands
+
+The project uses GitLab CI with custom templates:
+- Build and push are handled by the `build` and `push` commands from the `buildtool/build-tools` image
+- No local Makefile; builds run in CI pipeline
+
+## Release Process
+
+Releases use git-cliff for changelog generation:
+```bash
+git cliff -o CHANGELOG.md
+```
+
+Version is stored in `.version` as JSON: `{"version":"vX.Y.Z"}`
+
+## Conventional Commits
+
+This project uses conventional commits. Commit types:
+- `feat`: New features
+- `fix`: Bug fixes
+- `chore`: Maintenance tasks (deps, CI)
+- `docs`, `perf`, `refactor`, `style`, `test`: As standard