Resolve bundled skill paths from skill directory

[shiny-code-bot]

Summary

• teach the generic skills prompt to resolve bundled scripts/resources relative to the skill's SKILL.md directory
• document the same path-resolution rule in skills docs
• add a renderer regression test for the instruction

Validation

• cargo test -p code-core skills::render::tests::render_skills_section_resolves_relative_paths_from_skill_dir
• ./build-fast.sh