feat: make max concurrent subagents configurable via runtime config

Support configuring max_concurrent_subagents (2-4, default 3) through
config.configurable, with automatic clamping and dynamic prompt generation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/debug.py

@@ -32,6 +32,7 @@ logging.basicConfig(
     datefmt="%Y-%m-%d %H:%M:%S",
 )
 
+
 async def main():
     # Initialize MCP tools at startup
     try: