feat: add clean target to remove build artifacts

Adds standard clean target to remove binary, output files, and
profile directory.

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

Makefile

@@ -1,6 +1,6 @@
 BINARY_NAME=lambda
 
-.PHONY: it run profile explain graph
+.PHONY: it run profile explain graph clean
 
 it:
 	@ go build -o ${BINARY_NAME} ./cmd/lambda
@@ -20,3 +20,7 @@ graph:
 	@ go tool pprof -raw -output=profile/cpu.raw profile/cpu.prof
 	@ go tool pprof -svg profile/cpu.prof > profile/cpu.svg
 	@ echo "Profile graph saved to 'file://profile/cpu.svg'"
+
+clean:
+	rm -f ${BINARY_NAME} program.out
+	rm -rf profile/