test: sample fixture course

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

fixtures/courses/sample/lessons/04-exercise.md

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+# Adding numbers
+
+Functions take values and return values.
+
+## Exercises
+
+### Exercise 2.1 — `add`
+
+Implement `add(a, b)` so that it returns the sum of its arguments.
+
+## Hints
+
+<details><summary>Exercise 2.1 (add) — nudge</summary>
+
+Which operator combines two numbers?
+
+</details>
+
+<details><summary>Exercise 2.1 (add) — near-solution</summary>
+
+Return `a + b`.
+
+</details>