Trim third-party dependencies

go.sum, go.mod, and vendor/ are really quite large, now. This is a risk of not just bloat but security vulnerabilities. Figure out if anything can be trimmed (e.g. replaced with a more lightweight library or built-in implementation).

Start by reviewing go.mod, go.sum, and the vendor/ directory to inventory the current dependencies and their usage. Determine which dependencies can be removed or replaced with lighter libraries or built-in Go implementations; done means the dependency set is smaller without breaking the project.

Written by the indexing model from the issue text.