test/hil: clear HIL report up front on a fresh run

The report sidecar lives in a persistent dir (it survives the CI workspace
clean so accumulation works across run attempts). A full run is "fresh" and
must not merge prior state, but previously fresh only avoided *loading* the
json at merge time — if a fresh run crashed before writing the report, the
stale json/md from an earlier run lingered and a retry (fresh=False) could
merge it, or the always() upload could post it. Delete hil_report.json/.md at
the start of a fresh run so prior results can never leak.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>