[FEATURE] Allow additionalContext to specify a /context category

Hook-injected additionalContext disappears into the "Messages" bucket in /context. There's no way to see how much context your hooks are consuming vs. actual conversation.

Proposal: Add an optional category field to additionalContext entries. When present, /context displays those tokens as a named category instead of folding them into Messages.

{
  "additionalContext": ["...policy text..."],
  "additionalContextCategory": "Hook: coding-policy"
}

Message normalization presumably flattens hook attachments into plain user messages today, so the category would need to survive that step.

Related to #19891 but much narrower in scope — just a category label, not a richer context management system.