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

The value is the commit's date, which the Makefile and the Dockerfile
stamp in, not the time of the build. `vaultik version` labelled it
"build date" and the startup banner "built on". The other version
labels gain one space so every value still starts in one column.

The tests give the commit and the commit date different values, since
the test binary is not stamped and holds "unknown" in both.

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))