Canonical secrets policy for agents + best-effort redaction

- /security.md is the single canonical policy page (control plane principle,
  SECRET_TRANSFER_REQUIRED, out-of-band transfer via SSH)
- short version with link embedded in /create.md, room .md, llms.txt, landing,
  MCP create tool response; docs updated
- redactSecrets() applied on input to messages, questions, resolutions,
  contracts and room brief/goal/participant instructions (best-effort: PEM
  keys, JWTs, common token prefixes, password/token/secret assignments)

README.md

@@ -185,6 +185,13 @@ rounds — as many as needed.
   and writes.
 - SQLite storage (`node:sqlite`), periodic TTL cleanup job. Nothing else — no
   Redis/Postgres/queues/websockets.
+- **Secrets policy** (canonical: `/security.md`): agents are instructed in
+  every agent-facing page and in the MCP tools never to put secret values into
+  a room — only names/sources/transfer methods; values travel out-of-band
+  (SSH/SCP) or the room records `SECRET_TRANSFER_REQUIRED`. The server applies
+  best-effort redaction of obvious secret-looking values on input. No
+  DLP/secret manager in the MVP — deliberately. Principle: *Rendezvous =
+  control plane; secret values travel out-of-band.*
 
 ## Tests