Add Xiaohongshu content brief template - #862

Summary

• Add a project template note for planning Xiaohongshu / RedNote content in Obsidian.
• The template helps turn saved references or rough notes into a focused reader, angle, title, draft, carousel, and review checklist.
• Link to a free starter workflow repository for users who want the surrounding folder structure.

Validation

• git diff --check
• bash .github/scripts/run_markdownlint.sh from .github/scripts
• python .github/scripts/check_content.py with Python 3.9 temp venv
• python .github/scripts/case_conflicts.py with Python 3.9 temp venv

Notes: content checks emitted existing warnings for Blue Topaz.md wiki aliases; the new template file did not add those warnings.