feat(security): Yerin security-agent toolset (read-only)

New securityRegistry (separate from companionRegistry) with two read-only,
secret-free tools for the Yerin security agent:
- audit_log: query the redacted audit trail by actor_kind/actor_id
- agent_inventory: list agents + capabilities/scopes (explicit projection,
  never SELECT *, no token material)

Follows the existing createRegistry() pattern. Design + wiring roadmap in
docs/yerin-security-agent.md. Not yet seeded/exposed over MCP (left for review).

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

lib/ai/agent/tools/security/agent_inventory.js

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+import * as agents from '../../../../db/repos/agents.js';
+
+// Privilege inventory: which agents exist and what each is allowed to do.
+// Returns an explicit projection (never SELECT *) so token/secret columns can
+// never leak through this view even if the agents schema grows.
+export const agentInventoryTool = {
+  name: 'agent_inventory',
+  description: 'List every agent and its privilege level (capabilities + scopes). Use to audit who can read/suggest/write and to spot over-privileged agents. Never returns token material.',
+  input_schema: { type: 'object', properties: {} },
+  async handler(_args, _ctx) {
+    const rows = await agents.list();
+    const projected = rows.map(a => ({
+      id: a.id,
+      slug: a.slug,
+      name: a.name,
+      kind: a.kind,
+      model: a.model,
+      capabilities: a.capabilities || {},
+      scopes: a.scopes || {}
+    }));
+    return { agents: projected };
+  }
+};

lib/ai/agent/tools/security/audit_log.js

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+import * as audit from '../../../../db/repos/audit.js';
+
+// Yerin's window into the audit trail. Read-only. The audit repo already
+// redacts sensitive diff keys (token/password/api_key/secret/authorization)
+// at write time, so entries are safe to surface.
+export const auditLogTool = {
+  name: 'audit_log',
+  description: 'Review the security audit trail: who (which actor) did what, newest first. Filter by actor_kind (user/agent/cron/worker/system) and/or actor_id to investigate a specific principal.',
+  input_schema: {
+    type: 'object',
+    properties: {
+      actor_kind: {
+        type: 'string',
+        enum: ['user', 'agent', 'cron', 'worker', 'system'],
+        description: 'optional: only entries from this kind of actor'
+      },
+      actor_id: { type: 'string', description: 'optional: only entries from this actor id (uuid)' },
+      limit: { type: 'integer', description: 'max entries (default 50, max 200)' }
+    }
+  },
+  async handler({ actor_kind, actor_id, limit } = {}, _ctx) {
+    const capped = Math.min(Math.max(Number(limit) || 50, 1), 200);
+    const entries = await audit.listByActor({ actor_kind, actor_id, limit: capped });
+    return { entries };
+  }
+};

lib/ai/agent/tools/security/index.js

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+import { createRegistry } from '../../registry.js';
+import { auditLogTool } from './audit_log.js';
+import { agentInventoryTool } from './agent_inventory.js';
+
+// Yerin's security toolset — read-only observability, kept in its own registry
+// so the security agent gets security tools (not Dross's propose_change). A
+// future MCP server can expose this registry the same way companion-stdio.js
+// exposes companionRegistry. Roadmap for further tools: see docs/yerin-security-agent.md
+export const securityRegistry = createRegistry();
+securityRegistry.registerTool(auditLogTool);
+securityRegistry.registerTool(agentInventoryTool);