test/hil: accumulate HIL report across re-runs; post as sticky PR comment

hil_test.py persists results in a hil_report.json sidecar and regenerates
hil_report.md from it. A full run starts fresh; a re-run (--skip-board / -bt,
i.e. the .skip file) merges into the existing report so already-passed
boards/tests are preserved while only re-run cells update. The report dir is
configurable via HIL_REPORT_DIR.

build.yml: each HIL rig writes the report to a workspace-sibling dir that
survives the per-attempt workspace clean, and uploads it as an artifact. A new
hil-report job merges the rigs' reports into one sticky PR comment (marocchino)
with one table per rig.

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