Support skill short descriptions

[cbusillo]

Summary

• parse optional metadata.short-description from SKILL.md frontmatter into core skill metadata
• propagate the compact description through ListSkills while keeping full description for model-visible skill routing
• prefer the compact description in the TUI skills settings list when present

Validation

• cargo test -p code-core short_description -- --nocapture
• cargo test -p code-core render_skills_section_uses_full_description_for_model_context -- --nocapture
• cargo test -p code-tui skills_settings_view::tests -- --nocapture
• git diff --check
• ./build-fast.sh

Refs #72