docs: add CLAUDE.md project guide

[lpillmann]

Summary

• Adds CLAUDE.md to the permifrost repo so Claude Code and contributors immediately understand the project context
• Documents that this repo is a Gemma Analytics fork of the upstream project at https://gitlab.com/gitlab-data/permifrost
• Notes the relationship with tundri (object DDL) and permifrost (permission grants)

Changes

• New file: CLAUDE.md

What's in the CLAUDE.md

Section	Content
What is Permifrost	Fork origin, purpose, relationship to tundri
Development Commands	Install, run, test, lint
Architecture	Module overview table, execution flow
Environment Variables	Required Snowflake env vars
Key Files	pyproject.toml, VERSION, GEMMA_RELEASE.md
Release Process	bumpversion + twine workflow
Conventions	Source layout, never commit to main

Testing

No code changes — documentation only.

Linear Issue

Closes https://linear.app/gemma-analytics/issue/GEM-6/permifrost-repo-lacks-claudemd

🤖 Generated with Claude Code

[lpillmann]

@claude solve the conflicts for me and push a new reconciled version that is the most complete combination of the contributions

[lpillmann]

Closing as superseded — the CLAUDE.md content from this PR was already incorporated into main via PRs #14 and #15. The follow-up workflow guidance (superpowers plugin, TDD steps, plan files) is covered by PR #17.