Run $EDITOR through the shell in config edit (closes #315)

config edit used the whole of $EDITOR as the program name, so a value
with arguments such as "code --wait" or "emacsclient -t" failed with
"executable file not found". It now runs $EDITOR the way git does:
sh -c '<editor> "$@"' with the config path as a separate argument, so
the editor's own arguments and quoting work and a path with spaces
reaches it whole. A test runs config edit with an $EDITOR carrying an
argument on a config path that contains a space.

Model: opus-5-5

internal/cli/config.go

@@ -333,8 +333,15 @@ func newConfigEditCommand() *cobra.Command {
 				editor = "vi"
 			}
 
+			// Run $EDITOR through the shell, as git does, so a value
+			// with arguments such as "code --wait" works. The path
+			// goes in as an argument ("$@"), not into the command
+			// text, so a path with spaces or quotes arrives whole.
+			// sh -c takes the word after the command as $0, which
+			// is why editor appears twice.
 			//nolint:gosec // G204: launching the operator's own $EDITOR is the point
-			ed := exec.CommandContext(cmd.Context(), editor, path)
+			ed := exec.CommandContext(cmd.Context(),
+				"sh", "-c", editor+` "$@"`, editor, path)
 			ed.Stdin = os.Stdin
 			ed.Stdout = os.Stdout
 			ed.Stderr = os.Stderr