Add Best Practice

Add a new best practice rule to the correct best-practices document. Ensures no duplicates, assigns stable IDs, and follows the exact format of existing entries.

Step 1: Verify Working Directory

This skill runs from the brave-core (src/brave) directory. Best practices docs are at ./docs/best-practices/ and the index is at ./docs/best_practices.md.

Set DOCS_DIR="./docs/best-practices".

Step 2: Understand the Best Practice