Produce the initial version of the style guide that the rest of the epic builds on. A draft already exists, so this ticket is about reviewing and finalising it: confirming the set of rules is complete and that we agree on each one. As part of this work we should tag every rule with how it will be enforced

• tier A for rules a tool can enforce automatically;
• tier B for rules a tool can check but only advise on;
• tier C for rules that need AI or human judgment

For all those rules, we also need to indicate the target severity. This ticket is about creating the initial document, that will be discussed in another ticket.