Support Agent-level Model Selection for Task Tool

Problem

Currently, when using the Task tool to invoke subagents, the model specified in the Agent configuration file is ignored. All subagents use the main conversation’s model
regardless of their configuration.

Current Behavior

# agents/code-explorer.md                                                                                                                                                     
---                                                                                                                                                                           
name: code-explorer                                                                                                                                                           
model: mmodel  # MiniMax-M2.5 (0.2x cost)                                                                                                                                     
---                                                                                                                                                                           
                                                                                                                                                                              
When calling Task({subagent_type: "code-explorer", ...}), the agent uses the main conversation's model instead of mmodel.                                                     
                                                                                                                                                                              
Expected Behavior                                                                                                                                                             
The Task tool should respect the model field in Agent configuration, allowing different agents to use different models.                                                       
                                                                                                                                                                              
Use Case: Cost Optimization                                                                                                                                                   
 Task Type                                 │ Model                                    │ Cost                                     │ Benefit                                    
───────────────────────────────────────────┼──────────────────────────────────────────┼──────────────────────────────────────────┼──────────────────────────────────────────  
 Code Exploration                          │ MiniMax-M2.5                             │ 0.2x                                     │ Fast, cheap                                
 Architecture Design                       │ GLM-5                                    │ 0.5x                                     │ Deep reasoning                             
 Code Generation                           │ Qwen3.5-Plus                             │ 0.2x                                     │ Programming expert                         
 Documentation                             │ Kimi-K2.5                                │ 0.3x                                     │ Long text                                  
                                                                                                                                                                              
Benefits                                                                                                                                                                      
1. Cost Optimization: Use cheaper models for simple tasks                                                                                                                     
2. Specialization: Match model strengths to task types                                                                                                                        
3. Parallel Efficiency: Multiple agents with different models can work concurrently                                                                                           
                                                                                                                                                                              
Suggestion                                                                                                                                                                    
Add an optional model parameter to the Task tool, or respect the Agent configuration's model field.                                                                           
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Related Project: UltraWork - Multi-agent orchestration system https://github.com/paskaa/ultrawork-agent