Use gpg.conf's default-key when unlocker add pgp has no --keyid (closes #141)

getDefaultGPGKey took the default-key value from `gpgconf --list-options
gpg` as printed. gpgconf prints a string value with a leading `"` and
percent-escaped, so gpg rejected the key ID. The parsing moved into
parseDefaultGPGKey, which drops the `"` and percent-decodes the rest with
url.PathUnescape, which leaves `+` alone. A value that does not decode is
an error, not a fallback to the first secret key.

Rule suppressed: gosec G703 on the test's write of gpg.conf into the
GNUPGHOME directory newTestGPGKey made.

Model: opus-5-5

internal/cli/unlockers.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log"
 	"maps"
+	"net/url"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -66,12 +67,13 @@ func getDefaultGPGKey() (string, error) {
 
 	output, err := cmd.Output()
 	if err == nil {
-		for line := range strings.SplitSeq(string(output), "\n") {
-			fields := strings.Split(line, ":")
-			if len(fields) > 9 && fields[0] == "default-key" && fields[9] != "" {
-				// The default key is in field 10 (index 9)
-				return fields[9], nil
-			}
+		defaultKey, parseErr := parseDefaultGPGKey(string(output))
+		if parseErr != nil {
+			return "", parseErr
+		}
+
+		if defaultKey != "" {
+			return defaultKey, nil
 		}
 	}
 
@@ -98,6 +100,27 @@ func getDefaultGPGKey() (string, error) {
 	return "", errNoGPGSecretKeys
 }
 
+// parseDefaultGPGKey returns the default-key value in the output of
+// `gpgconf --list-options gpg`, or "" when gpg.conf sets none. gpgconf(1)
+// prints a string value with a leading `"` and percent-escaped.
+func parseDefaultGPGKey(gpgconfOutput string) (string, error) {
+	for line := range strings.SplitSeq(gpgconfOutput, "\n") {
+		fields := strings.Split(line, ":")
+		if len(fields) > 9 && fields[0] == "default-key" && fields[9] != "" {
+			// The default key is in field 10 (index 9)
+			keyID, err := url.PathUnescape(strings.TrimPrefix(fields[9], `"`))
+			if err != nil {
+				return "", fmt.Errorf("failed to decode gpgconf default-key %s: %w",
+					fields[9], err)
+			}
+
+			return keyID, nil
+		}
+	}
+
+	return "", nil
+}
+
 func newUnlockerCmd() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "unlocker",