- SecretBackend gains write/delete/list_names; FileSecretBackend + OpenBaoBackend
implement them (vault KV-v2 POST/DELETE/LIST); castle-api/secrets.py routes all
CRUD through the active backend (dashboard writes to the vault in openbao mode)
- add systemd exec_start_post (general; '-' prefix so a hook failure can't fail
the unit); castle-openbao runs an unseal.sh on boot using OPENBAO_UNSEAL_KEY
- tests: file write/read/list/delete round-trip
Verified live: write→vault→read→list→delete against OpenBao; seal→restart→
auto-unsealed. TLS hardening remains (deferred; gates cross-network/real secrets).