Add configurable public-release blocklist

[xichia]

Add an optional configurable blocklist for public-release validation.

This would help users catch private terms such as real university names, course codes, lecturer names, LMS hostnames, or internal project names before publishing a sanitized repo.

Acceptance criteria

• Add a simple blocklist file format
• make validate-public uses the blocklist if present
• Matches produce actionable errors or warnings
• Documentation explains how to use it
• Tests cover a blocked term

Suggested files

• studylib.py
• docs/public-release-checklist.md
• tests/