feat: support user-entered text in code blocks in bold

## Summary

In the book manuscript, user-entered text is shown in bold within command examples. For example:

```
$ **ls -al**
output here
```

The `$ ` prompt is regular weight, but `ls -al` (what the user types) is bold.

## Current Behavior

Our bash-formatted code blocks don't support inline bold styling:

```bash
$ ls -al
output here
```

Everything renders in the same weight.

## Desired Behavior

Find a way to visually distinguish:
- Prompt (`$`) - regular weight
- User input (`ls -al`) - bold or highlighted
- Output - regular weight

## Options to Explore

1. Custom code block component with markup parsing
2. CSS-based solution using syntax highlighting classes
3. Different approach (e.g., separate prompt/input/output blocks)

## Context

This is a migration issue - the book uses this convention throughout to help readers distinguish what they should type vs. what they'll see.