feat(skill): inject skill root directory path into prompt

[chmod777john]

Summary

• Inject the absolute path of the skill root directory into the prompt when loading a skill
• Allows AI to always know the base location of skill files
• Helps debug path issues even when skill authors mix absolute/relative paths
• Enables use of find/glob tools to locate files dynamically

Background

The previous approach of only converting specific directory patterns (scripts/, references/, etc.) was too limited:

• It didn't account for the variety of ways skill authors might structure their content
• Skill authors may use non-standard directory names
• Absolute/relative path confusion could still break skill loading

By injecting the skill root directory path directly into the prompt, AI agents can:

1. Always know where the skill files are located
2. Debug path issues independently
3. Use tools like find or glob to locate files dynamically

Test plan

• All existing tests pass
• Added new test test_skill_to_prompt_includes_root_directory