Add sfdupes dwim: scan / then print duplicate trees (closes #154)

sfdupes dwim, as sneak ordered: with no arguments it runs what scan / runs with no flags, default excludes on, then what trees runs, printing the duplicate trees largest first on stdout. Its last line, on stderr, says that sfdupes trees prints the report again. It needs no setup beyond what scan creates. SIGINT or SIGTERM during the scan stops it as it stops scan, exit 1, with no report; during the report a signal ends it at once, as it ends trees. A positional argument is a usage error, exit 2.

Model: opus-5-5

main.go

@@ -14,6 +14,7 @@
 //	sfdupes report > dupes.tsv
 //	sfdupes trees > dupetrees.tsv
 //	sfdupes info
+//	sfdupes dwim
 //	sfdupes --version
 //
 // See README.md for the complete specification.
@@ -45,6 +46,7 @@ const (
 	cmdReport = "report"
 	cmdTrees  = "trees"
 	cmdInfo   = "info"
+	cmdDwim   = "dwim"
 )
 
 // errNoSubcommand is returned by the root command when it is invoked
@@ -96,8 +98,8 @@ func main() {
 // exit code. It is the program's single exit point: the subcommands
 // return their errors instead of exiting, so every deferred cleanup —
 // above all closing the database, which checkpoints the SQLite WAL —
-// runs before the process ends. The report, trees and info subcommands
-// write their data to stdout.
+// runs before the process ends. The report, trees, info and dwim
+// subcommands write their data to stdout.
 func run(args []string, stdout, stderr io.Writer) int {
 	// A nil slice makes cobra fall back to os.Args, which would let a
 	// test binary's own flags reach the command tree.
@@ -201,7 +203,16 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		}),
 	}
 
-	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd)
+	dwimCmd := &cobra.Command{
+		Use:   cmdDwim,
+		Short: "Scan / with the default excludes, then print the duplicate-tree report",
+		Args:  cobra.NoArgs,
+		RunE: runE(func(ctx context.Context, _ []string) error {
+			return runDwim(ctx, "/", stdout)
+		}),
+	}
+
+	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd, dwimCmd)
 
 	return root
 }