fix: FollowTarget enum — actor_url no longer lies about holding a handle

FollowCommand.target is now FollowTarget (Identity|Handle) instead of
SocialIdentity. actor_url field always holds a URL; handles go through
FollowTarget::Handle. Adapter resolves handles explicitly. Type system
prevents misuse in future commands.

crates/domain/src/value_objects/social.rs

@@ -16,6 +16,12 @@ impl SocialIdentity {
     }
 }
 
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum FollowTarget {
+    Identity(SocialIdentity),
+    Handle(String),
+}
+
 #[derive(Clone, Debug)]
 pub struct SocialActor {
     pub identity: SocialIdentity,