docs: hand off unifying the HIL report's two artifacts

Four writers produce hil_report.md and three of them write no JSON - the no-boards
exit, the pool-guard fallback and _abandon_exit's text prepend. Those are exactly the
runs that failed, so hil_summary.py, which builds an agent's per-board verdicts from
the sidecar, sees nothing while a human reading the markdown sees the real story. The
scope note is markdown-only too, so a three-board PR run and a full run that lost 24
boards are indistinguishable in JSON.

Five tasks: put scope in the sidecar, render the markdown from the document, give the
two early-exit paths a document, make _abandon_exit set a caveat field instead of
prepending to a file it did not write, then pin the invariant that re-rendering the
JSON reproduces the markdown byte for byte. Split out because it is a hil_test.py
reporting refactor, and the abandon path runs while the interpreter is being torn
down - it deserves its own review.