Optional timestamps on messages, so replayed history keeps its time

[fzred]

Problem

When a client restores a session with history replay (session/load in v1, session/resume with replayFrom in the v2 draft), every message comes back without any notion of when it happened. A client can stamp live updates with its own clock, but once the session is reloaded (after a restart, or when opening an older conversation) that information is gone, and nothing in the protocol lets the agent hand it back.

The agents usually have it:

• Claude Code's transcript records a timestamp on every entry.
• Codex's app-server reports startedAt / completedAt / durationMs per turn (thread/turns/list) and startedAtMs / completedAtMs per item (thread/items/list).

Concrete use case: our client shows "Completed · 4m 56s · Oct 11, 14:31" under each turn. That works for turns that ran while the client was connected, and disappears for every replayed turn, which is exactly when users ask "when did I do this?". Grouping a long conversation by day, or showing how old a resumed session's last answer is, runs into the same gap.

Proposal

Let the agent attach an optional creation time to messages:

• v2: an optional createdAt (RFC 3339, like SessionInfo.updatedAt) on user_message, agent_message and agent_thought. This fits the patch semantics of the Message Updates RFD, which already leaves room for future message-level fields: replay sends it with the content, a live agent sends it on the first update for a messageId.
• v1: the same optional field on user_message_chunk / agent_message_chunk / agent_thought_chunk (meaningful on the first chunk of a messageId), or a documented _meta key if a v1 schema change is not wanted.

Agents that don't know the time omit it, and clients fall back to what they do today.

Open questions

• Is a message-level time enough, or should turn timing (start, end, duration, as Codex records it) be expressible too? Message times approximate it well (user message ≈ turn start, last agent message ≈ turn end), but cannot tell a finished turn from an interrupted one.
• Should tool calls carry the same field? Codex records start and completion times per item as well.

Related adapter issues

• Replay drops the times Codex records for each item and turn codex-acp#617
• Replayed messages carry no timestamp, though the transcript records one per entry claude-agent-acp#1307