Build `exercise` Rune

Summary

Practice problem with a prompt, optional hints, and a revealable solution. Encourages active learning.

Attributes

Name	Type	Default	Required	Description
`difficulty`	String	`'medium'`	No	`easy`, `medium`, `hard`
`points`	Number	—	No	Point value (for graded contexts)
`type`	String	`'open'`	No	`open` (free response), `code` (expects code), `multiple-choice`

Content Model

First paragraph/section → the problem prompt
## Hints or ## Hint heading → progressive hints (each child is one hint, revealed sequentially)
## Solution heading → the solution (hidden by default, revealed on click)
Code fences in the solution section get syntax highlighting

Transform Output

typeof: Exercise
Tag: <article>
Properties: difficulty, points, type
Refs: prompt (div), hints (ol, each li is a hint), solution (div, initially hidden)

Implementation Tasks

Create schema in runes/learning/src/tags/exercise.ts
Add RuneConfig entry in runes/learning/src/config.ts
Write CSS in packages/lumina/styles/runes/exercise.css
Import CSS in packages/lumina/index.css
Add hint revelation + solution toggle behaviors in packages/behaviors/
Write tests in runes/learning/test/tags/exercise.test.ts
Create inspector fixture

Dependencies

Hint revelation and solution toggle need JS — candidate for @refrakt-md/behaviors