Visual distinction between user input and assistant responses

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

Currently, user input and assistant responses are rendered in the same colour and style, making it very difficult to scan back through a conversation and tell them apart. This significantly hurts readability, especially in longer sessions.

### Proposed solution

Requested improvements (any of these would help):

1. Different text colour for user input vs assistant output (e.g., user input in cyan/green, responses in default white)
2. Background highlight on user input lines (subtle tint to separate them)
3. A visible separator line or label (e.g., ── You ── / ── Copilot ──) between turns
4. Bold or italic styling on user input to differentiate it at a glance

Why it matters:

- Hard to find where I typed something vs what was generated
- Especially painful in long multi-turn sessions
- /theme and terminal colour schemes don't help because the CLI uses the same styling for both
- This is a basic UX pattern in every chat interface (ChatGPT, Slack, Teams, etc.)

Environment: Windows Terminal, PowerShell, multiple themes tried.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by locating the CLI conversation-rendering entry point and compare how user input and assistant responses are currently styled. Choose and document one consistent visual distinction, then verify that turns are clearly separated in Windows Terminal and PowerShell across longer conversations.

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