Add sfdupes info: show the database's details (closes #117)

info prints the database's path and source, the sizes of the database
file and its -wal file, when the database file last changed, the schema
version and the record counts, one per line as a name, a tab and the
value. It opens the database as report does and shows an open and a
count phase on stderr. The schema version and the counts come from one
read-only transaction, the counts from one query.

seedDatabase now closes test databases out of WAL mode, as scan does;
otherwise info's own connection creates an empty -wal file. The open
phase's name and text became constants for the linter, and the scan
command moved to newScanCommand to keep newRootCommand short enough.

Judgement call: the lines are written after the last display ends, so they skip the display's stdout writer.

Model: opus-5-5

main.go

@@ -8,13 +8,14 @@
 // under 50 MiB, or of gigabyte-spaced 1 MiB samples when it is 50 MiB
 // or larger. scan maintains a persistent SQLite database of file
 // signatures (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite)
-// that the reporting subcommands read.
+// that the other subcommands read.
 //
 // Usage:
 //
 //	sfdupes scan [--workers N] [-x] PATH...
 //	sfdupes report > dupes.tsv
 //	sfdupes trees > dupetrees.tsv
+//	sfdupes info
 //	sfdupes --version
 //
 // See README.md for the complete specification.
@@ -44,6 +45,7 @@ const (
 	cmdScan   = "scan"
 	cmdReport = "report"
 	cmdTrees  = "trees"
+	cmdInfo   = "info"
 )
 
 // errNoSubcommand is returned by the root command when it is invoked
@@ -73,8 +75,8 @@ 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 and trees subcommands write
-// their data to stdout.
+// runs before the process ends. The report, trees and info 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.
@@ -148,6 +150,41 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	root.Flags().BoolVarP(&showVersion, "version", "v", false,
 		"print the version to stdout")
 
+	reportCmd := &cobra.Command{
+		Use:   cmdReport,
+		Short: "Read the scan database and print the file-level duplicates report",
+		Args:  cobra.NoArgs,
+		RunE: runE(func(ctx context.Context, _ []string) error {
+			return runReport(ctx, stdout)
+		}),
+	}
+
+	treesCmd := &cobra.Command{
+		Use:   cmdTrees,
+		Short: "Read the scan database and print the duplicate-tree report",
+		Args:  cobra.NoArgs,
+		RunE: runE(func(ctx context.Context, _ []string) error {
+			return runTrees(ctx, stdout)
+		}),
+	}
+
+	infoCmd := &cobra.Command{
+		Use:   cmdInfo,
+		Short: "Read the scan database and print its details",
+		Args:  cobra.NoArgs,
+		RunE: runE(func(ctx context.Context, _ []string) error {
+			return runInfo(ctx, stdout)
+		}),
+	}
+
+	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd)
+
+	return root
+}
+
+// newScanCommand builds the scan subcommand, with its --workers and -x
+// flags.
+func newScanCommand() *cobra.Command {
 	var (
 		scanWorkers int
 		scanOneFS   bool
@@ -172,27 +209,7 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	scanCmd.Flags().BoolVarP(&scanOneFS, "one-file-system", "x", false,
 		"do not cross filesystem boundaries")
 
-	reportCmd := &cobra.Command{
-		Use:   cmdReport,
-		Short: "Read the scan database and print the file-level duplicates report",
-		Args:  cobra.NoArgs,
-		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runReport(ctx, stdout)
-		}),
-	}
-
-	treesCmd := &cobra.Command{
-		Use:   cmdTrees,
-		Short: "Read the scan database and print the duplicate-tree report",
-		Args:  cobra.NoArgs,
-		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runTrees(ctx, stdout)
-		}),
-	}
-
-	root.AddCommand(scanCmd, reportCmd, treesCmd)
-
-	return root
+	return scanCmd
 }
 
 // checkScanWorkers rejects a scan --workers value below 1. That is a