Show timestamps next to messages in conversation view

### Describe the feature or problem you'd like to solve

When returning to a long-running Copilot CLI session after a break (e.g., coming back to the terminal after a few hours), there's no way to see **when** each message was sent. The conversation reads as a flat transcript with no time context.

This makes it hard to:
- Know when you last interacted with the session
- Understand the pacing of a conversation (was that exchange 5 minutes ago or 5 hours ago?)
- Orient yourself when resuming after a gap

The agent already receives `` with each user message, so the data exists — it's just not displayed.

### Proposed solution

Show a **timestamp** next to each user and assistant message in the terminal UI. Something like:

```
[07:23] > How does the G502 compare to the G502 X?

[07:23] The G502 X is essentially a modernised rebuild...

[13:42] > I just came back — when was my last message?
```

Options to consider:
- **Relative timestamps** for recent messages ("2m ago"), absolute for older ones ("07:23")
- **Gap indicators** — when there's a long gap between messages (e.g., >30 min), show a separator: `--- 6 hours later ---`
- **Configurable** — a setting in config.json to enable/disable, or choose format (relative / absolute / off)

### Example prompts or workflows

User works in a session in the morning, walks away, comes back in the afternoon. Currently they see a flat wall of text with no temporal context. With this feature they'd immediately see when the last exchange happened and how long they've been away.

### Additional context

The `` tag is already injected into each user message for the agent's benefit — this is about surfacing that same information in the UI for the human.