fix(agents): fall back to agents.defaults.model when agent has no model config

When an agent in agents.list lacks a model configuration, resolveAgentModelPrimary
now correctly falls back to cfg.agents.defaults.model instead of returning undefined.

This fixes embedded agents (like slug-generator) silently falling back to a hardcoded
DEFAULT_MODEL when the main agent omits the model block but global defaults are configured.

Fixes openclaw#24168

src/agents/agent-scope.test.ts

@@ -59,6 +59,38 @@ describe("resolveAgentConfig", () => {
     });
   });
 
+  it("falls back to agents.defaults.model when agent has no model configured", () => {
+    const cfgWithStringDefault: OpenClawConfig = {
+      agents: {
+        defaults: {
+          model: "anthropic/claude-sonnet-4",
+        },
+        list: [{ id: "main" }],
+      },
+    };
+    expect(resolveAgentModelPrimary(cfgWithStringDefault, "main")).toBe("anthropic/claude-sonnet-4");
+
+    const cfgWithObjectDefault: OpenClawConfig = {
+      agents: {
+        defaults: {
+          model: {
+            primary: "openai/gpt-5.2",
+            fallbacks: ["anthropic/claude-sonnet-4"],
+          },
+        },
+        list: [{ id: "main" }],
+      },
+    };
+    expect(resolveAgentModelPrimary(cfgWithObjectDefault, "main")).toBe("openai/gpt-5.2");
+
+    const cfgNoDefaults: OpenClawConfig = {
+      agents: {
+        list: [{ id: "main" }],
+      },
+    };
+    expect(resolveAgentModelPrimary(cfgNoDefaults, "main")).toBeUndefined();
+  });
+
   it("supports per-agent model primary+fallbacks", () => {
     const cfg: OpenClawConfig = {
       agents: {

src/agents/agent-scope.ts

@@ -144,14 +144,29 @@ export function resolveAgentSkillsFilter(
 
 export function resolveAgentModelPrimary(cfg: OpenClawConfig, agentId: string): string | undefined {
   const raw = resolveAgentConfig(cfg, agentId)?.model;
-  if (!raw) {
+  if (raw) {
+    if (typeof raw === "string") {
+      const trimmed = raw.trim();
+      if (trimmed) {
+        return trimmed;
+      }
+    } else {
+      const primary = raw.primary?.trim();
+      if (primary) {
+        return primary;
+      }
+    }
+  }
+
+  // Fallback to agents.defaults.model when agent has no model configured
+  const defaultModel = cfg.agents?.defaults?.model;
+  if (!defaultModel) {
     return undefined;
   }
-  if (typeof raw === "string") {
-    return raw.trim() || undefined;
+  if (typeof defaultModel === "string") {
+    return defaultModel.trim() || undefined;
   }
-  const primary = raw.primary?.trim();
-  return primary || undefined;
+  return defaultModel.primary?.trim() || undefined;
 }
 
 export function resolveAgentModelFallbacksOverride(