Add snapshot files to list a snapshot's files without restoring it (closes #435)

`vaultik snapshot files <snapshot-id> [path] [--json]` downloads and
decrypts the snapshot database, as restore does, and prints mode, size,
modification time and path for every file at or under the path. A path
that matches nothing is an error. It needs the private key. The mode is
in Go's file-mode notation, which the README spells out.

Named `files` because `ls` is already the alias of `snapshot list`.
The missing-key error no longer says "for restore", since this command
and deep verify return it too. The README opening now states the
private-key rule first.

Model: opus-5-5

internal/cli/app.go

@@ -38,10 +38,11 @@ const (
 	// prune, remote nuke) write the local index or the remote store. They
 	// hold the PID lock so that at most one runs at a time.
 	mutating lockMode = iota
-	// readOnly commands (info, snapshot list, snapshot verify, remote info,
-	// snapshot restore) do not write the local index or the remote store,
-	// so they run without the lock and are never blocked by a running
-	// mutator. restore writes only to the target directory it is given.
+	// readOnly commands (info, snapshot list, snapshot files, snapshot
+	// verify, remote info, snapshot restore) do not write the local index
+	// or the remote store, so they run without the lock and are never
+	// blocked by a running mutator. restore writes only to the target
+	// directory it is given.
 	readOnly
 )
 
@@ -310,7 +311,7 @@ func RunOperation(
 }
 
 // runVaultikApp runs the standard single-operation command lifecycle
-// shared by the snapshot list/purge/remove and remote nuke subcommands:
+// shared by the snapshot list/files/purge/remove and remote nuke subcommands:
 // resolve the config, then run op against the Vaultik instance through
 // RunOperation, reporting a failure prefixed with failMsg on stderr. mode
 // says whether the command takes the PID lock. jsonOutput marks a command