chore(logging): remove verbose subagent_start debug in executeSubagent

Reduce noisy backend logs while keeping the actual subagent_start event emission intact to avoid clutter in development and CI logs.

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Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: brandonkachen

backend/src/tools/handlers/tool/spawn-agent-utils.ts

@@ -340,13 +340,6 @@ export async function executeSubagent(
     onlyChild: isOnlyChild,
     parentAgentId: parentAgentState.agentId,
   }
-  withDefaults.logger.debug(
-    {
-      ...startEvent,
-      parentAgentId: parentAgentState.agentId,
-    },
-    'executeSubagent: Sending subagent_start event',
-  )
   onResponseChunk(startEvent)
 
   // Import loopAgentSteps dynamically to avoid circular dependency