feat: add help target as default Makefile goal

Added a help target that displays all available Makefile targets with descriptions.
Set help as the default goal so running 'make' without arguments shows usage information.

Makefile

@@ -1,6 +1,17 @@
 BINARY_NAME=lambda.exe
 
-.PHONY: build run profile explain graph docs
+.PHONY: help build run profile explain graph docs
+
+.DEFAULT_GOAL := help
+
+help:
+	@ echo "Available targets:"
+	@ echo "  build    - Build the lambda executable"
+	@ echo "  run      - Build and run the lambda interpreter (use TEST=<name> to specify sample)"
+	@ echo "  profile  - Build and run with CPU profiling enabled"
+	@ echo "  explain  - Build and run with explanation mode and profiling"
+	@ echo "  graph    - Generate and open CPU profile visualization"
+	@ echo "  docs     - Start local godoc server on port 6060"
 
 build:
 	@ go build -o ${BINARY_NAME} ./cmd/lambda