feat(core): promote top-level maxTurns to runConfig.max_turns at convert time

Wires the new top-level `maxTurns` field into the existing runtime configuration
pipeline so the value declared in agent frontmatter actually limits the agent's
turn budget at run time. When both the top-level `maxTurns` and the legacy
nested `runConfig.max_turns` are set, the top-level field wins (more specific
and matches the CC frontmatter shape upstream agent files use). When only the
nested field is set, behavior is unchanged.

Refs #4821 #4721 #4732

packages/core/src/subagents/subagent-manager.test.ts

@@ -1862,6 +1862,38 @@ System prompt 3`);
 
         expect(runtimeConfig.modelConfig).toEqual({});
       });
+
+      // --- CC 2.1.168 maxTurns top-level promotion ---
+
+      it('should populate runConfig.max_turns from top-level maxTurns', async () => {
+        const cfg: SubagentConfig = { ...validConfig, maxTurns: 42 };
+        const runtimeConfig = await manager.convertToRuntimeConfig(cfg);
+        expect(runtimeConfig.runConfig.max_turns).toBe(42);
+      });
+
+      it('should prefer top-level maxTurns over nested runConfig.max_turns', async () => {
+        const cfg: SubagentConfig = {
+          ...validConfig,
+          maxTurns: 99,
+          runConfig: { max_turns: 5 },
+        };
+        const runtimeConfig = await manager.convertToRuntimeConfig(cfg);
+        expect(runtimeConfig.runConfig.max_turns).toBe(99);
+      });
+
+      it('should fall back to nested runConfig.max_turns when maxTurns is unset', async () => {
+        const cfg: SubagentConfig = {
+          ...validConfig,
+          runConfig: { max_turns: 7 },
+        };
+        const runtimeConfig = await manager.convertToRuntimeConfig(cfg);
+        expect(runtimeConfig.runConfig.max_turns).toBe(7);
+      });
+
+      it('should leave max_turns undefined when neither is set', async () => {
+        const runtimeConfig = await manager.convertToRuntimeConfig(validConfig);
+        expect(runtimeConfig.runConfig.max_turns).toBeUndefined();
+      });
     });
 
     describe('mergeConfigurations', () => {

packages/core/src/subagents/subagent-manager.ts

@@ -853,6 +853,10 @@ export class SubagentManager {
 
     const runConfig: RunConfig = {
       ...config.runConfig,
+      // Top-level CC-style `maxTurns` wins over legacy nested
+      // `runConfig.max_turns`. Both are kept for backward compatibility, but
+      // when both are set, the top-level field is the authoritative source.
+      ...(config.maxTurns !== undefined ? { max_turns: config.maxTurns } : {}),
     };
 
     let toolConfig: ToolConfig | undefined;