feat(skills): group skills by category + surface when_to_use

[dcvz]

Summary

• Parse category and when_to_use from each skill's SKILL.md frontmatter (Skill gains both fields; defaults: category→tool, when_to_use→empty, so un-migrated skills still work).
• format_skills_xml now groups skills into <skill_group category="…"> blocks in a fixed order (tool, workflow, report, then any extras sorted) and surfaces <when_to_use> per skill — giving the agent the signal to choose/compose skills.
• cica stays taxonomy-agnostic: it renders whatever category a skill declares (unknown categories still shown); the vocabulary/rules live in the ai-skills repo, not here.

This is Phase A (skills foundation) of the River-style strategy. The companion ai-skills changes (Lean+ SKILL.md contract, curated 7-skill catalog, competitor-brief workflow) land in the ai-skills repo.

Test Plan

• cargo test --bin cica skills — 12 pass
• cargo clippy --all-targets -- -D warnings — 0
• cargo fmt --check — clean
• Backward-compat: skill with no category renders under tool; empty category: line also defaults to tool
• After ai-skills merges + router sync: skills appear grouped-by-category in the prompt

🤖 Generated with Claude Code