docs: Deduplicate validate-check helper

Keep a single Makefile validate-check target that prints the full
/metrics dump, highlights hit/miss fields, and curls /lancache-heartbeat.
Drop the duplicate README troubleshooting item and align the validation
chapter plus validate-config.yaml comments with that behavior.

Link: #29

docs/examples/validate-config.yaml

@@ -27,9 +27,10 @@
 #   SteamPrefill benchmark run -c 20 ...
 #
 # After the benchmark run, inspect with:
-#   make validate-check
-#   # or: curl -s http://localhost/metrics
-# Heartbeat (process up): curl -s -i http://localhost/lancache-heartbeat
+#   make validate-check   # full /metrics + hit/miss fields + /lancache-heartbeat
+#   # or, manually:
+#   curl -s http://localhost/metrics
+#   curl -s -i http://localhost/lancache-heartbeat   # GET, not HEAD
 #
 # Tweak sizes upward if you want to run very large workloads while still
 # exercising the disk tier (workload >> RAM is ideal for real disk testing).