feat: add dev-daemon target for background development mode (#1047)

* feat: add dev-daemon target for background development mode

Add a new make dev-daemon target that allows running DeerFlow services
in background mode without keeping the terminal connection.

Following the pattern of PR #1042, the implementation uses a dedicated
shell script (scripts/start-daemon.sh) for better maintainability.

- Create scripts/start-daemon.sh for daemon mode startup
- Add dev-daemon target to Makefile
- Each service writes logs to separate files (langgraph, gateway, frontend, nginx)
- Services can be stopped with make stop
- Use nohup for proper daemon process detachment
- Add cleanup on failure when services fail to start
- Use more specific pkill pattern to avoid killing unrelated nginx processes

* refactor: use wait-for-port.sh instead of hardcoded sleep in daemon script

* refactor: use specific nginx process pattern to avoid killing unrelated processes

* Revert "refactor: use specific nginx process pattern to avoid killing unrelated processes"

This reverts commit 4c369155bfc91ccce347876a8982f955fa039da8.

* refactor: use consistent nginx kill pattern across all scripts

* chore(daemon): add trap for cleanup on interrupt signals

* fix(daemon): pass repo root as positional argument to nginx command

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

Makefile

@@ -1,6 +1,6 @@
 # DeerFlow - Unified Development Environment
 
-.PHONY: help config check install dev start stop clean docker-init docker-start docker-stop docker-logs docker-logs-frontend docker-logs-gateway
+.PHONY: help config check install dev dev-daemon start stop clean docker-init docker-start docker-stop docker-logs docker-logs-frontend docker-logs-gateway
 
 help:
 	@echo "DeerFlow Development Commands:"
@@ -9,6 +9,7 @@ help:
 	@echo "  make install         - Install all dependencies (frontend + backend)"
 	@echo "  make setup-sandbox   - Pre-pull sandbox container image (recommended)"
 	@echo "  make dev             - Start all services in development mode (with hot-reloading)"
+  @echo "  make dev-daemon      - Start all services in background (daemon mode)"
 	@echo "  make start           - Start all services in production mode (optimized, no hot-reloading)"
 	@echo "  make stop            - Stop all running services"
 	@echo "  make clean           - Clean up processes and temporary files"
@@ -87,6 +88,10 @@ dev:
 start:
 	@./scripts/serve.sh --prod
 
+# Start all services in daemon mode (background)
+dev-daemon:
+	@./scripts/start-daemon.sh
+
 # Stop all services
 stop:
 	@echo "Stopping all services..."