Otherwise, why not just keep the password in an .env file, and state “grab the password from the .env file” in your Postgres skill?

Why not the filesystem?

I would create a local file (e.g. .env) in each project using postgres, then in my postgres skill, tell the agent to check that file for credentials.

I do also like to make skills on things that are more niche tools, like marimo (a very nice jupyter replacement). The model probably does known some stuff about it, but not enough, and the agent could find enough online or in context7, but it will waste a lot of time and context in figuring it out every time. So instead I will have a deep thinking agent do all that research up front and build a skill for it, and I might customize it to be more specific to my environment, but it's mostly the condensed research of the agent so that I don't need to redo that every time.