I came at this from the breaking side. Working out how to get into something is what teaches you where it leaks, and most of what I put up here is that pointed back the other way: here is what you are giving away without knowing, and here is what you can actually do about it.
Small tools. One binary, nothing to install. The pinned ones below are what I have going at the moment.
Whatever I build next, these stay the same:
Nothing leaves your machine. No uploads, no lookups, no telemetry. Not as a promise either: there is no network code in any of it, and a test reads the source and fails the build if any turns up.
Your file stays your file. Nothing overwrites what you pointed it at.
It tells you what it could not read. A short report is not a clean result. A tool that quietly gives up is worse than no tool, because it hands you confidence you have not earned.
No dependencies if I can help it. Usually that means writing the boring part myself instead of pulling in somebody else's.
Go most of the time. Rust when it earns its place. Whatever fits the job, really.
Spain.