Add CLAUDE.md for AI-assisted development

[aaronspring]

Summary

Add a CLAUDE.md file to help Claude Code and other AI assistants work more effectively with the climpred codebase.

This documentation file provides:

• Development commands: Common tasks like running tests (pytest with parallel options), linting (pre-commit, black, isort, flake8), formatting, and building docs
• Architecture overview: High-level explanation of core classes (HindcastEnsemble, PerfectModelEnsemble) and verification workflow components (Metrics, Comparisons, Alignment, Bias Removal, Bootstrap, References)
• Key design patterns: Dimension naming conventions (init, lead, member), lazy evaluation with dask, xarray-centric design, and composable verification
• Testing guidelines: How to run tests, use fixtures, and understand CI variants (minimum, maximum, upstream)
• Code style: Black, isort, flake8, and mypy configuration

This file follows the pattern recommended by Claude Code and similar tools (like Cursor's .cursorrules or Copilot's instructions) to reduce context-gathering time and help AI assistants be productive more quickly.

Type of change

• Improved Documentation

How Has This Been Tested?

• File created with comprehensive content based on analysis of the codebase
• Commands verified against CI configuration and pyproject.toml

Checklist

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the sphinx documentation, if necessary. (N/A - this is a standalone documentation file)
• CHANGELOG is updated with reference to this PR. (N/A - minor documentation addition)

🤖 Generated with Claude Code