First: anything that is a live incident
Before design, before style, before naming. Scan for credentials in source, private key blocks, connection strings with embedded passwords, and literals assigned to variables named password, secret or token.
If one is found, rotate at the provider before touching the file. A checklist that starts with formatting has already wasted the review.
Generated code puts secrets in examples, in comments and in the snippet a developer pasted to ask why the connection failed. Search the whole file, not the diff.