Disclose download-reference guidance progressively

[nzy1997]

The download-reference entry point mixed its core workflow with optional dependency setup, acquisition fallbacks, cache maintenance, and troubleshooting. This PR moves those conditional paths into linked references while keeping cite-key ownership, provenance, and KB restoration rules intact.

The change follows OpenAI's Rethinking skills and prompts for GPT-6 Astra: keep the router concise and disclose operational detail only when that path is needed. It is the download-reference slice split from #62 after review feedback.

Validation

• python3 scripts/validate_skills.py: 16 skills pass
• targeted resource and repository tests: 57 passed
• python3 -m pytest -q: 269 passed, 5 skipped
• the new test copies this skill without sibling skills and follows every relative Markdown reference
• git diff --check: clean

[GiggleLiu]

Polished on top of your commit (c05840d). Kept the move of dependency setup, acquisition fallbacks, cache restore, and troubleshooting into references/. Restored three things that were behavior changes rather than disclosure: the shared "Installed resources" block, the once-per-run LaTeX-source question, and the per-ref cite-key confirmation in single-shot mode (still skipped in bulk mode and for callers that pass keys — brainstorm-ideas and CLAUDE.md both document that contract). Also kept the --tex-source priority note and the kb_sync.py command in SKILL.md so the common path needs no extra file. Replaced the single-skill link test with tests/test_skill_relative_links.py, parametrized over every skill; it passes for all 16.