Add documentation about colored dots

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

The meaning of the colored dots in the left column (purple, green, blue, etc) is not explained

### Proposed solution

The CLI prefaces output with a colored dot. Purple appears to indicate user-facing output, green for tasks, etc. but the actual meaning is not explained anywhere.

Asking the CLI "what do the different colored dots in the left column of Copilot CLI indicate" does a doc search but returns no answers.

Add this information to the CLI docs.

### Example prompts or workflows

_No response_

### Additional context

_No response_