docs: remove SECRETS_RUNBOOK.md (internal-only)

This file was internal documentation that was inadvertently published
on the open-source repository. It described the team's secrets
rotation procedure including AWS Secrets Manager naming conventions,
ECS cluster/service names, and per-secret rotation impact tables -
reconnaissance-grade infrastructure detail that was never intended
for public consumption.

No actual secret values were ever in this file; only the procedure
for rotating them. The existing exposure has been cleaned up by
deleting the file here, and the file is now in the private repo's
.premium-paths Section 1 (CI guard fails if anyone tries to
reintroduce it on main).

Author: Chaitanya Laxman