Parse the age identity key once and accept every identity in it (closes #165)

Restore and verify --deep now parse the configured age secret key a
single time through a new internal helper that uses age.ParseIdentities
and hands every identity to age.Decrypt. A key file with several
identities (a whole age-keygen file) is fully accepted, so a blob
encrypted to any of its recipients decrypts, not just the first.

The helper is the first step of both commands, so a missing or
unparseable key now fails before anything is downloaded. Its error names
the configuration source (VAULTIK_AGE_SECRET_KEY or age_secret_key) and
never echoes the key value. config.extractAgeSecretKey and its silent
fallback are removed; the key is stored raw and parsed only where
decryption happens, so backup, list and prune are unaffected.

README, the restore help, and the missing-key error now show the key
read from a file with $(cat ...) rather than typed literally, keeping it
out of shell history, and say the variable may hold the whole key file.

Model: opus-4-8

internal/cli/snapshot_restore_test.go

@@ -0,0 +1,37 @@
+package cli //nolint:testpackage // exercises the unexported command constructor
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/spf13/pflag"
+)
+
+// TestRestoreCommandDoesNotTakeKeyAsArgument guards the fix for the age
+// key being echoed on the command line: restore must take the key only
+// from the environment, never as a flag value, and its help must show the
+// file-based form rather than a literal key that would land in shell
+// history.
+func TestRestoreCommandDoesNotTakeKeyAsArgument(t *testing.T) {
+	t.Parallel()
+
+	cmd := newSnapshotRestoreCommand()
+
+	cmd.Flags().VisitAll(func(f *pflag.Flag) {
+		lower := strings.ToLower(f.Name)
+		for _, banned := range []string{"key", "secret", "age", "identity"} {
+			if strings.Contains(lower, banned) {
+				t.Errorf("restore must not accept the key as a flag; found --%s", f.Name)
+			}
+		}
+	})
+
+	help := cmd.Long
+	if strings.Contains(help, "AGE-SECRET-KEY-") {
+		t.Error("restore help must not show a literal age private key to type")
+	}
+
+	if !strings.Contains(help, "$(cat ") {
+		t.Error("restore help should read the key from a file, e.g. $(cat ...)")
+	}
+}