fix(skill): document reference files in install guide

[Dikshj]

Summary

• Update docs/install.md so the Skills directory row documents the complete payload: SKILL.md plus references/*.md.
• Add regression coverage that the install guide mentions references/*.md.
• Strengthen skill install coverage to assert all packaged reference markdown files are copied into the installed skill directory.

Fixes #622.

Tests

• python -m pytest tests\test_skill_command.py tests\test_opencode_skill.py -q
• python -m pytest tests -q (fails in this local Windows environment due to unrelated missing dependencies loguru/yt_dlp, symlink privileges, and existing Xiaoyuzhou script test failures)