fix(skill-creator): don't crash report generator on empty history

[mvanhorn]

Summary

generate_html() in skills/skill-creator/scripts/generate_report.py calls max(history, ...) unconditionally to pick the best iteration to highlight. When history is empty, max() raises ValueError: max() iterable argument is empty, so the report generator crashes instead of producing a valid report.

Closes #417.

Why this matters

The report is meant to be rendered incrementally (the script even supports auto_refresh), so it can legitimately be asked to render before the first iteration result exists — a live "run just started" state, a partial result, or any caller that wants a valid page before evaluation begins. Every other part of generate_html already tolerates an empty history (the query-discovery block is guarded by if history:); the best-iteration highlight was the one spot that still assumed at least one iteration, turning an expected early state into a crash.

Fix

Guard the empty case before the max() calls:

if not history:
    best_iter = None
elif test_queries:
    best_iter = max(history, key=lambda h: h.get("test_passed") or 0).get("iteration")
else:
    best_iter = max(history, key=lambda h: h.get("train_passed", h.get("passed", 0))).get("iteration")

best_iter is only read inside the for h in history: row loop, which does not run when history is empty, so None is safe. The non-empty path is unchanged.

Testing

Reproduced against the current file: generate_html({"history": []}) raised ValueError: max() iterable argument is empty before the change and returns a valid ~5.8KB HTML document after it. A non-empty history still emits the best-row highlight (verified), and python3 -m py_compile generate_report.py passes.