feat: add help target to document available commands

Adds help target that displays all available Make targets and their
descriptions, improving discoverability.

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

Makefile

@@ -1,6 +1,17 @@
 BINARY_NAME=lambda
 
-.PHONY: it run profile explain graph clean
+.PHONY: help it run profile explain graph clean
+
+help:
+	@ echo "Available targets:"
+	@ echo "  it       - Build the lambda binary"
+	@ echo "  run      - Build and run with sample input (default: simple.txt)"
+	@ echo "  profile  - Build and run with CPU profiling enabled"
+	@ echo "  explain  - Run with explanation mode and profiling"
+	@ echo "  graph    - Generate CPU profile visualization"
+	@ echo "  clean    - Remove build artifacts"
+	@ echo ""
+	@ echo "Usage: make run TEST=<sample-name>"
 
 it:
 	@ go build -o ${BINARY_NAME} ./cmd/lambda