Permanent revoke and rekey retired the live pubkey without retaining it,
so historical checkpoints and off-box anchors became permanently
unverifiable. Add append-only agent_keys (public material only); retire
on revoke/rekey; verify against the fingerprint-named historical key.
Document a dedicated checkpoint-signer identity in the runbook.
Add requirements.txt (cryptography>=43.0.0,<45) and docs/PROD_RUNBOOK.md
requiring SIGIL_SEAL_PASS and SIGIL_ANCHOR on separate storage. Document
info findings R9/R12/R13 in DESIGN; leave R10 domain separation as a
versioned follow-up (not backward-safe).
