AgentAuthorship

Per-act agent-authorship metadata — rides in kb_events.metadata, NOT the payload, so it is
invisible to projections (and thus affinity math) by construction.

confidence is required whenever authorship is supplied (it is non-Option): a caller either
attaches authorship — and must grade its confidence — or attaches none at all (the whole
[ActContext::authorship] is None). The other fields are optional context.

{
    "confidence": "tentative",
    "model": "string",
    "persona": "string",
    "rationale": "string",
    "reasoning": "string"
}