refactor: optimize task tool parameter order and improve task tracking

- Reorder task tool parameters to prioritize description first for better usability
- Add tool_call_id injection for better task traceability
- Use tool_call_id as task_id in executor for consistent tracking
- Simplify event messages by removing redundant task_type field
- Update task examples in prompt to reflect new parameter order

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/subagents/executor.py

@@ -284,16 +284,19 @@ class SubagentExecutor:
 
         return result
 
-    def execute_async(self, task: str) -> str:
+    def execute_async(self, task: str, task_id: str | None = None) -> str:
         """Start a task execution in the background.
 
         Args:
             task: The task description for the subagent.
+            task_id: Optional task ID to use. If not provided, a random UUID will be generated.
 
         Returns:
             Task ID that can be used to check status later.
         """
-        task_id = str(uuid.uuid4())[:8]
+        # Use provided task_id or generate a new one
+        if task_id is None:
+            task_id = str(uuid.uuid4())[:8]
 
         # Create initial pending result
         result = SubagentResult(