Delete knowledge base

Deletes the collection and every document only it holds, including those documents' storage. A document that another collection also holds survives with its membership here removed. The graph layer does not survive: every graph-derived row for the collection is destroyed with it: nodes, edges, custom entity and relationship types, package subscriptions, and the whole claim ledger of extraction runs, extraction sources, attribute claims and fact resolutions. This is strictly more destructive than DELETE /{kb_id}/graph, which refuses the ledger without discard_evidence=true; deleting the collection takes no such confirmation. Export an archive first if the evidence matters. Every user's package personas for the collection go too: a copy nobody edited is deleted and its sessions continue with the default persona; a copy someone edited stays as that person's own persona, with no graph wired.