Add prompt for sanitizing a private notes repo for public release

[xichia]

Add a reusable LLM prompt for creating a sanitized public framework candidate from a private notes repo.

Acceptance criteria

• Add prompts/sanitize-for-public-release.md
• Prompt follows docs/public-release-checklist.md
• Prompt requires proposal-first behavior before deleting or rewriting content
• Prompt removes or replaces real course-derived content with synthetic/demo content
• Prompt requires make validate-public and make pre-release
• Prompt warns not to weaken validation

Suggested files

• prompts/sanitize-for-public-release.md
• docs/public-release-checklist.md
• LLM_GUIDE.md