Claircore Documentation

Here's various codebase conventions that don't have dedicated pages:

• URLs URLs in code should be annotated with a //doc:url directive comment. See the the internal/cmd/mdbook-injecturls command for documentation on how the preprocessor works. The list of keywords isn't an allowlist, so an invocation like the following should list the ones actually used in the documentation using a command like git grep injecturls -- :/docs/*.md.