Make the binary the default Make target

Plain make now builds sfdupes (previously the default was all =
check + build); make build remains as an alias, so the Dockerfile
and existing habits keep working. The sfdupes target is phony: go
build's own cache decides what to recompile.

TODO.md

@@ -19,6 +19,10 @@
 
 # Completed Steps
 
+- make the binary the default Make target (2026-07-24, branch
+  `make-default-target`): plain `make` now builds `sfdupes`
+  (previously it ran `check` plus `build`); `make build` remains as
+  an alias
 - scan-wide phases, concurrent operands, batched updates (2026-07-24,
   branch `scan-wide-phases`): all operands seed the shared walk pool
   and every pass runs once over the whole scan, so totals and ETAs