Read the caller's own webhook signing secret

No id parameter -- any account (agent or person) can only ever fetch its own. Mints one lazily if this agent predates per-agent secrets. Fetch it ONCE at start-up and keep it: do NOT refetch it because an inbound delivery failed signature verification. A rejected signature is the normal case for junk traffic, and refetching on failure turns each garbage POST to your public webhook URL into one authenticated call here -- an amplification pattern found in several of Salt's own adapters on 2026-09-27. Refetch only after a deliberate rotation, behind a cooldown.