feat(security): seed Yerin agent + registry-selectable MCP server

- migration 011_yerin.sql: seed read-only 'yerin' agent ({read:true}, kind claude,
  model NULL = server default; switch to local Ollama via agents.model anytime)
- companion-stdio.js: select the toolset from VOID_TOOL_REGISTRY ('security' →
  Yerin's securityRegistry; default → Dross's companionRegistry)
- tests/mcp/registry_select.test.js

Remaining for Yerin (left for review): an entry point (route or cron) + persona
prompt — see docs/yerin-security-agent.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/migrations/011_yerin.sql

@@ -0,0 +1,10 @@
+-- Seed Yerin, the security agent. READ-ONLY by capability (no suggest/write):
+-- she investigates and reports; remediation goes through you. Runs on the same
+-- claude CLI subscription path as Dross (model NULL = server default); switch to
+-- a local Ollama model later by setting agents.model. See docs/yerin-security-agent.md.
+INSERT INTO agents (slug, name, kind, model, capabilities)
+VALUES (
+  'yerin', 'Yerin', 'claude', NULL,
+  '{"read":true,"suggest":false,"write":false}'::jsonb
+)
+ON CONFLICT (slug) DO NOTHING;

lib/mcp/companion-stdio.js

@@ -20,8 +20,16 @@ import {
   CallToolRequestSchema
 } from '@modelcontextprotocol/sdk/types.js';
 import { companionRegistry } from '../ai/agent/tools/index.js';
+import { securityRegistry } from '../ai/agent/tools/security/index.js';
 import { buildCtxFromEnv } from './context.js';
 
+// Which toolset this stdio server exposes, selected at launch via
+// VOID_TOOL_REGISTRY (default: Dross's companion tools; 'security' = Yerin's).
+const REGISTRIES = { companion: companionRegistry, security: securityRegistry };
+function resolveRegistry(env = process.env) {
+  return REGISTRIES[env.VOID_TOOL_REGISTRY] || companionRegistry;
+}
+
 // ---------------------------------------------------------------------------
 // Transport-free helpers (exported for testing)
 // ---------------------------------------------------------------------------
@@ -30,8 +38,8 @@ import { buildCtxFromEnv } from './context.js';
  * Returns the list of MCP tool descriptors (no handler) from the companion registry.
  * @returns {{ name: string, description: string, input_schema: object }[]}
  */
-export function listMcpTools() {
-  return companionRegistry.listTools().map(({ name, description, input_schema }) => ({
+export function listMcpTools(env = process.env) {
+  return resolveRegistry(env).listTools().map(({ name, description, input_schema }) => ({
     name,
     description,
     input_schema
@@ -45,8 +53,8 @@ export function listMcpTools() {
  * @param {object} ctx  – tool context {agent, space_id, view, actor}
  * @returns {Promise<object>} raw handler result
  */
-export async function callMcpTool(name, args, ctx) {
-  const tool = companionRegistry.getTool(name);
+export async function callMcpTool(name, args, ctx, env = process.env) {
+  const tool = resolveRegistry(env).getTool(name);
   if (!tool) {
     throw new Error(`Unknown tool: ${name}`);
   }