chore: add Claude Code workflows and fix Ruby version

[vishaltps]

Summary

• Fix Ruby version to 3.3.6 to resolve native gem ABI issues
• Add Claude Code project instructions and custom development workflows

Problem

• Ruby 3.4.0dev causes sqlite3/nokogiri native extension failures
• No standardized workflows for commits, PRs, code reviews, and testing

Solution

• Lock Ruby to stable 3.3.6 via .ruby-version
• Add minimal CLAUDE.md with git workflow rules and skill references
• Create 6 custom skills in .claude/skills/ for consistent development

Changes

• .ruby-version - locks project to Ruby 3.3.6
• CLAUDE.md - project instructions (git workflow, planning mode, tech stack)
• .claude/skills/principal-thinking.md - principal engineer mindset
• .claude/skills/commit-style.md - commit message format
• .claude/skills/create-pr.md - PR template and process
• .claude/skills/review-code.md - code review checklist
• .claude/skills/write-tests.md - test writing standards
• .claude/skills/apply-solid.md - SOLID principles (pragmatic)

Testing

• bundle install succeeds with Ruby 3.3.6
• bundle exec rspec passes (154 examples, 0 failures)

Checklist

• Code follows project patterns
• Self-review completed
• Tests pass locally