code-modernization: every proof_pack.py run erases the person's sign-off in VERIFICATION.md

Plugin: code-modernization 1.0.0, scripts/proof_pack.py (line ~867) and commands/modernize-verify

Every run of proof_pack.py writes "signoff": {"name": "", "role": "", "date": "", "decision": ""} and a blank Sign-off table into VERIFICATION.md/.json. A person's signature recorded there is wiped by the next run, including a scoped re-run (proof_pack.py <system> <module>).

Repro: run verify, have the owner fill the Sign-off block, run proof_pack.py <system> again (e.g. after re-verifying one module). The block is blank; nothing records that it was ever signed.

Expected: keep an existing signoff when the verified evidence is unchanged, or move it to a history list (signoffs: [{..., covers: <commit/date>}]) and mark it as covering an older verification, instead of erasing it. Today users have to copy signatures into another file (we keep them in the brief) to make them durable.