Stream execution events

Reconnect to a durable agent turn: replay-and-tail its Server-Sent Events. A turn that streams over an expired watch lease ends with `await` / `awaitReason: "detached"` while the execution keeps running; pass the last seen SSE `id:` as `after` to rejoin it. The session owner is resolved from the execution's own persisted record — its conversation for a top-level turn, its private child thread for a subagent run — and the optional `conversationId` is used only for a turn that persists no record of its own. Replays buffered events strictly past the `after` cursor, then live-tails if the turn is still running. Each event carries an SSE `id:` line — a durable row seq, or (unified vocabulary) a composite `<seq>.<subIndex>` when one durable row translates to multiple events. Pass that id verbatim after any disconnect (tab reload, sleep, stream timeout) to resume without missing or duplicating events. Replayed and live-tailed frames use the unified execution vocabulary.