Add helper to mark notes reviewed

[xichia]

Add a lightweight helper for day-to-day study use that updates review metadata after a study session.

Acceptance criteria

• Add a simple script such as mark_reviewed.py
• Script accepts a note ID
• Script updates last-reviewed
• Script proposes or updates review-after using documented cadence
• Script validates after editing
• README documents the workflow

Suggested files

• mark_reviewed.py
• README.md
• tests/