Label the commit date "commit date" in version and banner (closes #409)

vaultik version printed globals.CommitDate, the date of the commit the binary was built from, labelled "build date", and the startup banner called it "built on". Both now say "commit date". The test sets Commit and CommitDate to distinct values and checks the label, padding and value of every report line.

Model: opus-5-5

internal/cli/app.go

@@ -90,7 +90,7 @@ func setupGlobals(
 // blank line. Used both from the fx hook (for subcommand invocations) and
 // from the root cobra Run handler (for `vaultik` with no subcommand).
 func writeStartupBanner(w *ui.Writer, startTime time.Time, shortCommit string) {
-	w.Bannerf("%s %s by %s (commit %s, built on %s) starting up at %s.",
+	w.Bannerf("%s %s by %s (commit %s, commit date %s) starting up at %s.",
 		globals.Appname, globals.Version, globals.Author,
 		shortCommit, globals.CommitDate,
 		startTime.Format(time.RFC3339))