Feature request: show timestamps on messages in conversation

Problem

During long sessions (30-90+ minutes with background tasks, CI waits, parallel agents), there's no way to see when each message or tool call happened. This makes it hard to:

• Understand how long a step actually took
• Correlate chat events with external logs (CI runs, API calls, background processes)
• Review session history and reconstruct timing

Proposed solution

Add optional timestamps to messages in the conversation view:

• A setting like showTimestamps: true in settings.json
• Subtle HH:MM on each message (like Slack hover timestamps)
• Tool call results already track duration internally — surface it

Context

This came up during a session with 6 parallel worktree agents, 120+ API calls, and CI waits spanning ~90 minutes. Impossible to tell when things happened without checking external logs.