fix(agents): stop advertising custom agents to orchestrators

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/custom-agent-orchestrator-visibility.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, spyOn, test } from "bun:test"
+import { createBuiltinAgents } from "./builtin-agents"
+import * as shared from "../shared"
+
+const TEST_DEFAULT_MODEL = "anthropic/claude-opus-4-6"
+
+describe("createBuiltinAgents custom agent visibility", () => {
+	test("#given runtime custom agents #when orchestrator prompts are built #then custom agents are not advertised for automatic delegation", async () => {
+		//#given
+		const fetchSpy = spyOn(shared, "fetchAvailableModels").mockResolvedValue(
+			new Set(["anthropic/claude-opus-4-6", "openai/gpt-5.4"])
+		)
+
+		try {
+			//#when
+			const agents = await createBuiltinAgents(
+				[],
+				{},
+				undefined,
+				TEST_DEFAULT_MODEL,
+				undefined,
+				undefined,
+				[],
+				[
+					{
+						name: "backend-engineer",
+						description: "Custom backend specialist",
+					},
+				]
+			)
+
+			//#then
+			expect(agents.sisyphus.prompt).not.toContain("backend-engineer")
+			expect(agents.hephaestus.prompt).not.toContain("backend-engineer")
+			expect(agents.atlas.prompt).not.toContain("backend-engineer")
+		} finally {
+			fetchSpy.mockRestore()
+		}
+	})
+})