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)

packages/core/src/markdown.ts

@@ -62,6 +62,8 @@ export interface MarkdownInput {
   resolvedQuestions: QuestionView[];
   contractMarkdown: string;
   contractVersion: number;
+  /** Short secrets policy (links to the canonical /security.md). */
+  secretsPolicyShort: string;
 }
 
 export function renderRoomMarkdown(i: MarkdownInput): string {
@@ -73,6 +75,8 @@ export function renderRoomMarkdown(i: MarkdownInput): string {
   parts.push('');
   parts.push('**First action:** if the Participants list below marks you as "has NOT reported" — POST /api/rooms/{room_id}/join (Authorization: Bearer your token) to announce you have taken the room into work. The other side sees it.');
   parts.push('');
+  parts.push(i.secretsPolicyShort);
+  parts.push('');
   parts.push(`- **Goal:** ${i.goal || '(not set)'}`);
   if (i.brief) parts.push(`- **Brief:** ${i.brief}`);
   parts.push(`- **Status:** ${i.status}`);