security: add a runbook for responding to a compromised admin key

[Idaonoli]

Scope

SECURITY.md covers vulnerability disclosure, but there's no operational runbook for an issuer whose admin key is compromised — which contracts have an upgrade path that could help, which don't and would need a full redeploy, and what circuit-breaker/pause actions are available as an immediate stopgap.

Acceptance criteria

• Add a compromised-admin-key runbook to SECURITY.md or a new docs/incident-response.md, covering immediate mitigation (freeze/pause) and longer-term recovery per contract