Fix transfer rough edges: value-name hint, identity warnings, single-profile export

- --help now shows <PATH.toml> instead of the format-agnostic <PATH>.
- Export warns (naming the affected profiles) that identity files
  aren't included, only their local paths; import warns per-profile
  when an identity path doesn't resolve on the importing machine,
  expanding a leading ~/ the same way ssh.rs does so that check isn't
  a false positive for tilde paths.
- transfer now takes an optional profile-name positional (same shape
  as OpenArgs.name) so -e/--export can target a single profile instead
  of always dumping every saved one; combining it with -i/--import is
  rejected with a new TransferNameWithImport error.

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

src/cli.rs

@@ -188,11 +188,14 @@ pub struct WipeArgs {
 
 #[derive(Args)]
 pub struct TransferArgs {
-    /// Export all saved profiles to a file.
-    #[arg(short, long, value_name = "PATH")]
+    /// Name of the profile to export; every saved profile if omitted. Not used for import.
+    pub name: Option<String>,
+
+    /// Export saved profiles to a file.
+    #[arg(short, long, value_name = "PATH.toml")]
     pub export: Option<String>,
 
     /// Import profiles from a file.
-    #[arg(short, long, value_name = "PATH")]
+    #[arg(short, long, value_name = "PATH.toml")]
     pub import: Option<String>,
 }