feat: implement subagent delegation for task management

- Introduced the `delegate_task` tool, allowing the main agent to spawn child AIAgent instances with isolated context for complex tasks.
- Supported both single-task and batch processing (up to 3 concurrent tasks) to enhance task management capabilities.
- Updated configuration options for delegation, including maximum iterations and default toolsets for subagents.
- Enhanced documentation to provide clear guidance on using the delegation feature and its configuration.
- Added comprehensive tests to ensure the functionality and reliability of the delegation logic.

tools/__init__.py

@@ -156,6 +156,13 @@ from .code_execution_tool import (
     EXECUTE_CODE_SCHEMA,
 )
 
+# Subagent delegation (spawn child agents with isolated context)
+from .delegate_tool import (
+    delegate_task,
+    check_delegate_requirements,
+    DELEGATE_TASK_SCHEMA,
+)
+
 # File tools have no external requirements - they use the terminal backend
 def check_file_requirements():
     """File tools only require terminal backend to be available."""
@@ -261,5 +268,9 @@ __all__ = [
     'execute_code',
     'check_sandbox_requirements',
     'EXECUTE_CODE_SCHEMA',
+    # Subagent delegation
+    'delegate_task',
+    'check_delegate_requirements',
+    'DELEGATE_TASK_SCHEMA',
 ]