Show [name] for optional positionals, allow filtering import by name too

The custom --help renderer showed every positional as <name> regardless
of whether it was actually required, which was misleading for open and
transfer (both take an optional profile name). positional_args() now
checks Arg::is_required_set() and renders [name] for an optional one.

transfer's name positional previously only worked with --export and was
rejected outright when combined with --import. It now filters --import
the same way: 'porthole transfer <name> -i file.toml' imports just that
one profile out of the file instead of everything in it, erroring with
a new TransferProfileNotFound if the file doesn't contain it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/cli.rs

@@ -188,7 +188,7 @@ pub struct WipeArgs {
 
 #[derive(Args)]
 pub struct TransferArgs {
-    /// Name of the profile to export; every saved profile if omitted. Not used for import.
+    /// Restrict export/import to just this profile; every profile in scope if omitted.
     pub name: Option<String>,
 
     /// Export saved profiles to a file.