Leave empty files out of report and trees (closes #178)

Removing an empty file frees nothing, yet every `.gitkeep`,
`__init__.py` and empty log formed one `report` group, and directories
holding only empty files were reported as 0-byte duplicate trees. Now
`report` writes no group of size 0 and `trees` writes no group whose
trees hold 0 bytes. An empty file still counts toward the digest of a
tree that holds other files. No option, as approved on the issue. The
README's definition-of-done test now expects `empty1` and `empty2`
nowhere.

Model: opus-5-5

TODO.md

@@ -29,9 +29,6 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
-- The usage line of `scan` no longer ends in `[flags]`. It is the line
-  `README.md` shows under "Subcommands" (2026-11-11,
-  https://git.eeqj.de/sneak/sfdupes/issues/169)
 - `report` and `trees` leave out empty files. `report` writes no group of size
   0, and `trees` writes no group whose trees hold 0 bytes. An empty file still
   counts toward a tree's digest (2026-11-11,

internal/sfdupes/cli.go

@@ -215,13 +215,10 @@ func newScanCommand() *cobra.Command {
 	short := "Walk trees and synchronize the scan database"
 
 	scanCmd := &cobra.Command{
-		Use: cmdScan + " [--workers N] [-x] [--no-default-excludes] PATH...",
-		// The Use line already names every flag, so cobra must not
-		// append its generic "[flags]" to it.
-		DisableFlagsInUseLine: true,
-		Short:                 short,
-		Long:                  short + ".\n\n" + defaultExcludesHelp(runtime.GOOS),
-		Args:                  cobra.MinimumNArgs(1),
+		Use:   cmdScan + " [--workers N] [-x] [--no-default-excludes] PATH...",
+		Short: short,
+		Long:  short + ".\n\n" + defaultExcludesHelp(runtime.GOOS),
+		Args:  cobra.MinimumNArgs(1),
 		PreRunE: func(cmd *cobra.Command, _ []string) error {
 			return checkScanWorkers(cmd, scanWorkers)
 		},

internal/sfdupes/cli_test.go

@@ -714,34 +714,6 @@ func TestRunHelp(t *testing.T) {
 	}
 }
 
-func TestRunScanUsageLine(t *testing.T) {
-	t.Parallel()
-
-	// README §Subcommands shows this usage line for scan, and the help
-	// and the usage error both print it.
-	const want = "\n  sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...\n"
-
-	cases := map[string][]string{
-		"help":          {cmdScan, "-h"},
-		"missing paths": {cmdScan},
-	}
-
-	for name, args := range cases {
-		t.Run(name, func(t *testing.T) {
-			t.Parallel()
-
-			var stdout, stderr bytes.Buffer
-
-			run(args, &stdout, &stderr)
-
-			if !strings.Contains(stderr.String(), want) {
-				t.Errorf("run(%v) stderr = %q, want the usage line %q",
-					args, stderr.String(), want)
-			}
-		})
-	}
-}
-
 func TestRunVersion(t *testing.T) {
 	t.Parallel()