fix: derive code-grader score from assertions when score field is absent

[christso]

Bug

When a code-grader script returns assertions but omits score, the score defaults to 0 — even if all assertions pass:

// packages/core/src/evaluation/graders/code-grader.ts
const score = clampScore(typeof parsed?.score === 'number' ? parsed.score : 0);

So {"assertions": [{"text": "ok", "passed": true}]} → score 0. Wrong.

Expected Behaviour

• score present → use it (explicit override for weighted/non-binary scoring)
• score absent + assertions present → derive as passing / total
• Neither present → score 0

Impact

Scripts currently must redundantly compute score themselves to avoid getting 0:

passed = sum(1 for a in assertions if a["passed"])
score = passed / len(assertions) if assertions else 0.0
print(json.dumps({"score": score, "assertions": assertions}))

With the fix, the score line becomes optional boilerplate — scripts can just return assertions and the framework computes the score.