Feature Request: Include timestamps in conversation messages visible to the model

Summary

Messages in the conversation context do not include timestamps visible to the model. The model cannot determine when a message was sent, how much time has passed between messages, or the temporal distance to earlier parts of the conversation.

Use Case

1. Temporal awareness in long sessions: In multi-hour sessions, the model cannot distinguish "this was said 5 minutes ago" from "this was said 3 hours ago." This affects prioritization and relevance judgments.

1. Time-sensitive context: When external events have timestamps (notifications, API responses), the model cannot correlate them with conversation flow.

1. Session continuity: After compaction, temporal context is lost entirely. Even approximate timestamps would help the model reconstruct "what happened when."

Current Workaround

Sending periodic time signals via keep-alive messages (e.g., [14:40] every 15 minutes). This provides approximate temporal anchoring but is imprecise and relies on external tooling.

Proposed Solution

Include a timestamp field (ISO-8601 or Unix epoch) in each message that the model can access during generation. Even a coarse granularity (minute-level) would be valuable.

Relation to Other Requests

This complements #34184 (context window usage visibility). Together, they enable models in long-running sessions to understand both how full their context is and when things happened within it.