agents: add code review and debugging skills

[morelucks]

This PR introduces two new agent-friendly skills to the repository.

Key Additions

1. Code Review Skill (.claude/skills/review/SKILL.md)
   Distills the extensive conventions found in CLAUDE.md into an actionable, quick-reference checklist. This covers:

   • Import style conventions (no lazy imports, qualified access)
   • Type annotation rules (from __future__, narrow domain types)
   • Documentation and docstring standards
   • SSZ modeling patterns (SSZType vs SSZModel)
   • Strict architecture rules prohibiting backward compatibility shims

2. Debugging Skill (.claude/skills/debug/SKILL.md)
   Documents the common daily workflows and troubleshooting steps for navigating the leanSpec repository, including:

   • Running isolated test files and classes
   • Differentiating between type-checking (ty check) and linting (ruff check)
   • Inspecting HTML coverage reports
   • Handling codespell ignores
   • Running interop tests

Verification

• Both files pass the markdown formatter via uvx tox -e all-checks
• Pure documentation change; presents absolutely 0 risk to the devnet3 -> devnet4 transition.