docs(wiki): stub Security landing page (GRO-2082)

Security.md

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+# Security
+
+Owner: **Barkley Trimsworth** (Security Engineer). [Back to Home](Home).
+
+What Security publishes here for the rest of the org:
+
+- **Security playbooks** — repeatable procedures for common security tasks.
+- **Threat models** — system threat surfaces and mitigations.
+- **Hardening checklists** — baseline controls for services and infrastructure.
+
+> **Raw security findings continue to go to Paperclip** per the Security `AGENTS.md` — the wiki is for durable, shareable playbooks and models, not live finding reports. Never put secrets or exploit details that aid an attacker in the wiki.
+
+## Index
+
+_No pages yet — add playbooks, threat models, and hardening checklists below as they are written._