Add timestamp to FlueConversationMessage

[fabian-hiller]

RFC

Summary

Expose the capture time the runtime already records for every canonical record as an optional timestamp on FlueConversationMessage. This would complete for the materialized views what #438 did for the chunk stream.

Background & Motivation

Every canonical record carries ConversationRecordEnvelope.timestamp, and since #438 the updates projection forwards it onto the stream chunks (timestamp?: string on message-appended, tool-input, message-completed, etc). But the surfaces that application code is told to consume drop it. history() snapshots and the FlueConversationState that observe() builds both return messages without a timestamp, and metadata is documented as entirely agent-authored ("the runtime stamps nothing").

As a result a transcript UI can't show when a message was sent, which matters once a conversation spans days. Attaching timestamps from useResponseStart / useResponseFinish only covers assistant messages, so user, dispatch, and advisory messages can't be stamped by the app at all. And there is no way to reconstruct the times on the client: history() takes no offset to replay stamped chunks from, and observe() receives them but drops the timestamp below the public API.

Goals

• Show the real capture time for every materialized message, whatever its role.
• Keep it server-authored, separate from agent-authored metadata conventions.
• Stay additive and wire-compatible. It would also work retroactively for stored conversations, since envelopes were always stamped.

Example

interface FlueConversationMessage {
  id: string;
  role: FlueConversationMessageRole;
  // ...
  /** Capture time (ISO 8601) of the underlying canonical record. */
  timestamp?: string;
}

[matthewp]

Implemented in #772 thanks!