@copilot wrong way of generating functions.txt. should use a hook similar to how we generate module-tree.txt

Originally posted by @nathanjmcdougall in #1624 (comment)

Rather than summarizing code objects e.g. functions, classes, and modules with a manually maintained reference material, it is always better to have a "one source of truth" mindset and use the docstring via automation hooks/packages to extract the relevant information and keep it in-sync. For example, the docs/module-tree.txt design.

This information should go into a new skill called usethis-agents which provides a directory for all the usethis-skill-* skills (again, maintained via a prek hook according to this very principle!), as well as instructions for maintaining AGENTS.md.