feat(skills): add /debug skill

[morelucks]

Closes #202

Summary

Add a new /debug Claude skill under .claude/skills/debug/ as the top-level entry point for debugging gean failures and regressions.

This skill is intended to cover both:

• multi-client/devnet debugging
• gean-only debugging

It gives Claude a structured workflow for:

• starting from evidence
• reducing the failing scope
• classifying the likely subsystem
• routing into existing skills when appropriate
• ending with a concise debug summary

What changed

Added:

• .claude/skills/debug/SKILL.md
• .claude/skills/debug/references/debug-paths.md
• .claude/skills/debug/references/debug-workflows.md

The new skill:

• defines /debug usage examples
• adds routing and stop conditions
• points to existing skills instead of duplicating them
• maps common symptoms to the relevant gean subsystems
• documents investigation flows for:
  • existing devnet logs
  • fresh multi-client reproductions
  • package-local Go test failures
  • spec fixture mismatches
  • XMSS / Rust FFI failures
  • checkpoint sync / persistence issues