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. The table
quotes a path that holds a non-printable character.

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/ui/ui.go

@@ -17,6 +17,9 @@
 // Count, Percent) return ANSI-colored strings the caller composes into
 // the message body. When color is disabled (non-TTY output or NO_COLOR
 // set) all formatters return plain text.
+//
+// QuoteUnprintable is the same escaping without colour, for plain tables
+// that print paths from the snapshot database.
 package ui
 
 import (
@@ -227,17 +230,17 @@ func (w *Writer) Hex(s string) string {
 		short = s[:hexAbbrevLen] + "..."
 	}
 
-	return w.paint(ansiCyan, sanitize(short))
+	return w.paint(ansiCyan, QuoteUnprintable(short))
 }
 
 // Snapshot colorizes a snapshot ID (full, no abbreviation).
 func (w *Writer) Snapshot(id string) string {
-	return w.paint(ansiCyan+ansiBold, sanitize(id))
+	return w.paint(ansiCyan+ansiBold, QuoteUnprintable(id))
 }
 
 // Path colorizes a filesystem path.
 func (w *Writer) Path(p string) string {
-	return w.paint(ansiBlue, sanitize(p))
+	return w.paint(ansiBlue, QuoteUnprintable(p))
 }
 
 // Size colorizes a byte count using humanize.Bytes.
@@ -312,14 +315,15 @@ func (w *Writer) paint(color, s string) string {
 	return color + s + ansiReset
 }
 
-// sanitize returns s unchanged when every rune in it is printable, and a
-// double-quoted, backslash-escaped form (\n, \x1b, …) otherwise. The
+// QuoteUnprintable returns s unchanged when every rune in it is printable,
+// and a double-quoted, backslash-escaped form (\n, \x1b, …) otherwise. The
 // string value formatters escape their argument through this before
 // painting: identifiers, paths and symlink targets they render come from
 // the snapshot database, which is not trusted, and escaping must happen
 // before colour is applied — the painted result already contains the
 // escape codes the raw text would otherwise be indistinguishable from.
-func sanitize(s string) string {
+// Plain tables call it directly.
+func QuoteUnprintable(s string) string {
 	for _, r := range s {
 		if !unicode.IsPrint(r) {
 			return strconv.Quote(s)