secret list gives each secret's own creation and last-update times (closes #142)

secret list and secret list --json gave the time of the listing as every
secret's creation and last-update time, because Secret.LoadMetadata set
both to the current time. It now reads file modification times, so
listing still needs no passphrase: the last update is when the secret's
current file was written, and the creation is when its oldest version's
value.age was written. A move to another vault gives the copied files
the times of the originals.

GetSecretObject no longer loads the times, so secret get keeps its error
for a missing current file; secret list calls LoadMetadata itself.

Judgement call: the creation time comes from value.age, not from the
oldest version's directory, whose time changes when the next add
rewrites that version's metadata.age.

Model: opus-5-5

internal/cli/version.go

@@ -65,7 +65,9 @@ func VersionCommands(cli *Instance) *cobra.Command {
 		Use:   "promote <secret-name> <version>",
 		Short: "Promote a specific version to current",
 		Long: "Updates the current symlink to point to the specified " +
-			"version without modifying timestamps",
+			"version. The versions' own times are left unchanged; secret " +
+			"list then gives the time of the promote as the secret's last " +
+			"update.",
 		Args: cobra.ExactArgs(2), //nolint:mnd // secret-name and version args
 		ValidArgsFunction: func(
 			cmd *cobra.Command, args []string, toComplete string,