feature: introduce "Event" handling

[jpmcb]

Some orgs pump AI agent sessions into persistent Kafka topics. We should explore implementing a event.Publisher interface for Kafka.

type Event struct {
    Type      Type
    Key       string        // partition key (root hash for session grouping)
    Node      *merkle.Node
    Timestamp time.Time
}

type Publisher interface {
    Publish(ctx context.Context, event Event) error
    Close() error
}

We can then build a pkg/event/kafka/ implementor of the Publisher interface.

[linear]

PCC-106