Remove CLAUDE.md (#43)

The file is mostly stale: it references infrastructure that doesn't exist (`edit-plugin-skills` skill, `/release` skill, `.claude/skills/release/`, `sdk-versions.json`). The non-stale content lives in better places:

- "Don't fix skill bugs here, fix upstream" → already in CONTRIBUTING.md
- Related repos (Cursor, Codex, canonical skill) → already in README.md
- Repository structure → derivable from `ls`
- Version-match invariant between SKILL.md and plugin.json → belongs in a release-time check, not in session memory

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: starfleeth