test: sample fixture course

Co-Authored-By: Claude Opus 5.5 <[email protected]>

fixtures/courses/sample/lessons/01-intro.md

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+# Introduction
+
+Welcome to the sample course. It exists so tests have something small to load.

fixtures/courses/sample/lessons/02-video.md

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+# Transcript
+
+A two-second test pattern plays. Nothing is said.

fixtures/courses/sample/lessons/03-code.md

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+# Code along
+
+Run the script in the section files:
+
+```sh
+sh hello.sh
+```

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>