Skills: relative file paths in SKILL.md are not resolved

Summary

When a skill's SKILL.md references a file using a relative path (e.g. references/style-guide.md), the LLM has no way to resolve it because it doesn't know the skill's install directory. This causes the agent to use Search/Glob instead of Read, which fails to find the file.

Reproduction

1. Create a skill with a references/ subdirectory containing a file
2. Reference it in SKILL.md with a relative path: ` See references/style-guide.md `
3. Install and invoke the skill
4. The agent searches for the file and gets 0 results

Expected behavior

The skill system should resolve relative paths in SKILL.md to absolute paths before injecting the prompt, or expose the skill's base directory to the LLM so it can construct the correct path.

Workaround

Instruct the LLM to locate the file dynamically: "Read the file references/style-guide.md located in this skill's directory"