Add time context to assistant prompts (timestamp or time-since-last-message)

Description

The assistant currently receives the date but has no sense of time passing between messages. This leads to awkward interactions where the assistant has no idea if 30 seconds or 30 minutes have passed since the last exchange — resulting in poorly timed follow-ups and no ability to reason about durations.

Proposal

Add a lightweight time signal to each message, either:

• A timestamp: [2026-04-11 14:32:07] (~8 tokens)
• Time since last message: [3m] (~1 token)

The minimal option ([3m]) is preferred as it's essentially free in terms of token cost while solving the core problem.

Use cases

• Knowing whether a long-running process (file copy, build, deploy) is likely still in progress
• Avoiding premature follow-ups ("how's that going?" after 20 seconds)
• Better conversational pacing and temporal reasoning
• Providing more useful time-based estimates and suggestions

Impact

Near-zero token cost, significant improvement in conversational awareness and interaction quality.