Add prompt for logging mistakes during study

[xichia]

Add a reusable prompt that helps convert a conversational mistake description into the structured five-field Mistake Log format.

Acceptance criteria

• Add prompts/log-a-mistake.md
• Prompt extracts date, source, mistake, correction, and tags
• Prompt proposes the block before editing
• Prompt preserves visibility and source-risk
• README and prompts/README.md reference the prompt, if a prompt index exists

Suggested files

• prompts/log-a-mistake.md
• README.md
• LLM_GUIDE.md