The <location> field in the skills XML prompt was being XML-escaped,
converting <embedded:research>/SKILL.md to <embedded:research>/SKILL.md.
When the LLM tried to use read_file with this escaped path, it would fail.
Changes:
- Remove escape_xml() call from location field in prompt.rs
- Add fallback handling for escaped paths in try_read_embedded_skill()
- Add tests for both prompt generation and read_file handling
Fixes embedded skill loading for agents like butler running outside the g3 repo.