fix(agents): fall back to agents.defaults.model when agent has no model config (#24210)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 0f272b1027
Co-authored-by: bianbiandashen <16240681+bianbiandashen@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/commands/agent.ts

@@ -2,7 +2,6 @@ import {
   listAgentIds,
   resolveAgentDir,
   resolveEffectiveModelFallbacks,
-  resolveAgentModelPrimary,
   resolveSessionAgentId,
   resolveAgentSkillsFilter,
   resolveAgentWorkspaceDir,
@@ -21,6 +20,7 @@ import {
   modelKey,
   normalizeModelRef,
   resolveConfiguredModelRef,
+  resolveDefaultModelForAgent,
   resolveThinkingDefault,
 } from "../agents/model-selection.js";
 import { runEmbeddedPiAgent } from "../agents/pi-embedded.js";
@@ -372,29 +372,9 @@ export async function agentCommand(
       sessionEntry = next;
     }
 
-    const agentModelPrimary = resolveAgentModelPrimary(cfg, sessionAgentId);
-    const cfgForModelSelection = agentModelPrimary
-      ? {
-          ...cfg,
-          agents: {
-            ...cfg.agents,
-            defaults: {
-              ...cfg.agents?.defaults,
-              model: {
-                ...(typeof cfg.agents?.defaults?.model === "object"
-                  ? cfg.agents.defaults.model
-                  : undefined),
-                primary: agentModelPrimary,
-              },
-            },
-          },
-        }
-      : cfg;
-
-    const configuredDefaultRef = resolveConfiguredModelRef({
-      cfg: cfgForModelSelection,
-      defaultProvider: DEFAULT_PROVIDER,
-      defaultModel: DEFAULT_MODEL,
+    const configuredDefaultRef = resolveDefaultModelForAgent({
+      cfg,
+      agentId: sessionAgentId,
     });
     const { provider: defaultProvider, model: defaultModel } = normalizeModelRef(
       configuredDefaultRef.provider,