Add optional tools, model, and maxTurns to prompt modules

Why: User overrides of a subagent profile need a place on the shared contract, and empty fields must stay omitted so older modules keep their meaning.
Evidence: go test ./... passed, including the new prompt_module_subagent golden.
Impact: Existing modules serialize the same. Frontend and UI contracts still need a copy of the type.
Refs: feature/agent-harness-p3b
Co-authored-by: Cursor <cursoragent@cursor.com>

conformance_test.go

@@ -60,6 +60,14 @@ func sampleRun() Run {
 	}
 }
 
+func TestGoldenPromptModule(t *testing.T) {
+	golden(t, "prompt_module_subagent", PromptModule{
+		ID: "agent.sub.explore", Layer: LayerAgent, Version: 1,
+		Title: "探索子代理", Body: "只读检索。",
+		Tools: []string{"knowledge_search", "web_search"}, Model: "lightweight", MaxTurns: 8,
+	})
+}
+
 func TestGoldenDomain(t *testing.T) {
 	golden(t, "message", sampleMessage())
 	golden(t, "run", sampleRun())